You're both introverts who are bad at flirting. His mistake was not being more forthcoming. Your mistake was treating his girlfriend as his wife. You're allowed to flirt back.
I think he was flirting with you, in his own awkward way, and you rejected the flirt, in order to spare his feelings. You negated his statement instead of, for example, playing with it. Here's a quick off the cuff example reply: "There's kind of a chicken and egg question there. Do you start off bf/gf and THEN take such pictures, or do you take such pictures and then become bf/gf?"
See what I did there? I'll outline:
1) No rejection.
2) Made the topic abstract.
3) Hinted at possibilities without pressing one way or the other.
Hey! Thank you for the response, your perspective is much appreciated!
I don't really think either of us is bad at flirting, mostly because he was flirting with me all weekend practically (and yes, I innocently flirted back most of the time-- which was really just friendly banter). I think he's just scared to be vocal about his feelings, and went with a cryptic/passive route (perhaps disguised as flirting). He should have been more forthcoming, as you mentioned. I seem to always be left with responsibility of clearing the air.
The girlfriend/wife distinction is hard for me, because regardless, I still want to protect his honor (and her heart) because I care about him, and I'm not the type of person to aid in unfaithfulness. I actually feel a little insulted by it, because it seems like the easy road (as apposed to actually expressing his feelings, and handling the situation).
Instead of speaking, he's done things like the aforementioned taking pictures of wildflowers: saving me pineapple starburst because I said I liked them (even though they are his favorite), helping me carry things, etc. We also have these notes at the end of camp that you can write to others to encourage them. He wrote me one, and it was extremely genuine and touching. All signs point to the same conclusion, am I wrong?
I do, however, see what you mean by the rejection bit-- and I know that's the last thing I should ever do to a 9.

I
was really caught off guard and confused, but if "being like siblings" is rejection for him, then logically he
does have non-platonic feelings right??
I guess my question is, what now? Is there anyway to revisit this? I've thought about texting him and telling him I lied about seeing us as siblings, but that's kind of risky.
I'm sorry to say I apparently don't have permission to access this page likely due to me only being here a short time. Could you copy and paste the scenario here?
Sure, here you go!
Foreword: I just returned from a weekend away that I always enjoy, my church's summer youth camp. Over 2,000 high school students went this year, myself among close to 250 adult leaders. The weekend is always exhausting (both physically and emotionally) but so, so rewarding. I have a group of girls that I meet with every week during the school year, so it is very much like a mentoring thing, but I always meet a few new ones at camp. I could go on forever about the stories of students, and the way God moves, but what I fell out of bed this morning really needing to write about, is more in regards to some leader dynamics.
My fellow adult leaders are amazing, they are some of the most quality humans I've met-- perhaps this is because it requires some emotional and spiritual maturity to volunteer with high school students, or to be vulnerable about your own faults for the sake of their growth. Whatever it is, I've met some very interesting people in the (going on) five years I've been a smallgroup leader.
The Point: One such person is a guy who is part of my team (there are thirty different ones, each with around eight leaders) that I've become friends with over the past year. He's a 9w1, healthy enough to be funny and speak his mind, but does tend to listen more than talk. There is chemistry there, and we clearly enjoy each other's company, but he's never asked me out or been forthcoming. Then, in the spring, I asked him about the mushroom foraging that he does and we ended up planning to go together. It was really fun and we had good conversation, but I just didn't feel like he was interested-- never once held a branch back for me, and kind of kept a lot of physical distance between us the whole time. Afterwards, I just thought maybe he's not interested. The next few days though, included texting me pictures of wildflowers I had mentioned I liked, and inviting me to a bonfire at his house.
After that, things kind of tapered off. He was traveling, and summer came and I wrote it off. Then, on Friday when I saw him again, he was all charm and warmth. Quick to help me with things, joking and flirting with me. When we were about to get on the bus to leave for camp, he mentioned that he just started dating someone (that I know). This would not have confused or bothered me at all, if he hadn't just been flirting with me. I expressed my happiness for them, though, and thought that would be the end of it. The rest of the weekend he was still warm, if slightly less flirtatious, until a mutual good friend took a picture of us. She posted it on Instagram, and we were commenting what a nice picture it was (more for color and composition than anything else) he said, "yeah, it would be better if we were dating though." How does one respond to that?!
So like an idiot (and out of confusion and fear), I said something to the effect of "nah, we're like siblings." I think I said it to protect him, and prevent him from emotionally cheating on his new girlfriend, but also to protect myself from the embarrassment of admitting I kind of like him (when he's dating someone else). His response was to say nothing, and kind of look away. It felt like disappointment (I could be wrong, but) like saying that was his passive way of admitting he wants to date me? So what does that mean? That he's just having second thoughts about his girlfriend, or am I just reading way too into this...? I do know men have been intimidated by me in that past, so that could be it.
The thing I'm working on with my therapist at the moment, is trusting my intuition (because I was invalidated a lot growing up). Do you think my instincts are correct in this scenario? I'd love to hear thoughts from some 9's!