I'm an ISFP and I THINK I'm a type 9. I often see type 9s described as the 'peacemakers'. We enjoy harmony and like things to go smoothly. That's not to say we don't have very strong opinions and values about certain things. In my experience, I'm very easygoing, happy to go with the flow unless something clashes with a specific desire or value. Then I will stand up for my beliefs or thing I want. I think if I'm not that bothered about something why make a fuss? I think there is a similarity in that OUTWARDLY type 9s can appear like a blank slate who go with the flow. I guess ISFPs often appear as being quite mild and pleasant and understanding which can unfortunately be mistaken for being a doormat. It's like there are only two types of people: those who get walked over and those who walk over others. But I don't think that's the case, I think 9s strike a middle balance of knowing which fight to fight and favouring harmony and peace over petty disagreements.
As an ISFP I DO place huge importance on knowing or finding who I am and my identity, but I often feel like I don't know. So that links in with what you said about nines not being sure of who they are, I think it just takes time. For me it's like I know fragments of who I am, there are some definites....I just don't know the picture
I just read something about 9s that says 'at their healthiest, Nines are awake to themselves and to life, engaged with life and others, and move forward in their right action and calling.' I can see where ISFP fits in, although I must say that I did initially think 9 was linked to Fe however I am very sensitive to outward environments. I like to maintain peace both inside and outside and disrupting either causes an imbalance so I am aware of others moods, feelings, tone of voice etc in that sense. Also I think Fi awareness of our own feelings helps us relate to others as we can imagine how something would make us feel