People are going to speculate, but I think the problem is there are so many "fan theories" on there on what is happening in the TV series but it's all really just remains speculation because the show itself is not really written in a fair way if such speculations are actually true. It's why a lot of people are upset with this episode and have been upset with other episodes. In this case, there's not really any TRUE signifiers that we're meant to read Arya this way; it's all people trying to connect possible data points after the fact.
On the other hand, look back at earlier seasons. Maybe Oberyn choosing to champion Tyrion seems surprising on first pass (for non-book readers) but in hindsight if you rewatch the episodes it's all really clear that, based on small comments/dialogue lines and historical data we are given about old resentments, that this is going to happen. Or step outside of this series -- watch Shyamalan's "The Sixth Sense" where a last minute revelation puts the entire movie in a whole new light. On first pass, most people don't seem it coming; on second watch, it's all there, you see the movie entirely differently and a smart viewer who hasn't fallen prey to misdirection can put it together. (Or Nolan's "The Prestige" -- the secret to the original version of the Transporting Man is clearly telegraphed through the film but you have been misdirected to think something different.) That's actually good writing -- something that scans as surprising initially and afterwards now appears inevitable.
This stuff about Arya is kind of fascinating to consider, but there's nothing in the show that really makes it seem like what we were supposed to walk away with. If GRRM goes this direction in his final books, maybe it will be more clarified; but right now it just seems like sloppy writing and people trying to go back after the fact and somehow justify it all. We are given literally nothing about the Faceless Man in terms of his actual intentions or even that he gives a shit about the Night King, even after this turn of events; going back for a rewatch won't reveal anything new. Right now this all scans as "Well, this is what happened, so this must have been what the writers intended." Or maybe the writers either didn't have these ideas, or maybe they just did a poor job in including enough real anchor points. Who knows? The show has definitely been much more explicit about how petty the fight for the throne is in the face of the Night King's threat... so I'm really hoping they don't completely ignore that now and try to make a happy resolution resulting in someone winning the throne, like a conventional fantasy story.
I dunno. There have been more "fan theories" about GoT than you could shake a flaming stick at, some of them highly ridiculous. Others are "possible" because nothing excludes them, but we really don't know what's going on the Faceless Man. In the show, he scanned to me as both surprised and pleased that Arya walked away, which could have been a mistake by the actor; but we knew as an audience all along that she was using the assassin program to implement her own plan, rather than truly being committed to their overt agenda. Maybe that means FM would also be aware of that, but as far as the writing goes, I can't tell. Again, maybe GRRM will deal with it better if this is the intent. If this is what is meant to happen, it should be surprising (which is what it was) but also appear inevitable afterwards (which it doesn't).
