At that point (around the time of the acquisition), I guess one could say there was significant overlap between FB and IG users, and that FB had more users than IG.

Yes, that's what I meant. The point I was trying to make was that just knowing server side scripting was a necessary but not sufficient condition to create a hit photo sharing app at that time, because there weren't many options on the camera at the time, and the filters were what drove the growth at that time.