Yes, in the games, that's true. But there's no reason they couldn't make a movie solely focused on Altair's or Ezio's or Connor's story. Each of their stories contains a self-contained conflict between the Assassins and Templars that is easily understood and digested without knowing any of the present-day stuff at all. The present-day stuff is just fluff that gets in the way of those self-contained stories and makes everything more complicated than it needs to be. Yes, you can provide a framing, as the games do, but there's no actual need to as all of those work by themselves and capture what people really love about the series.
I mean, it's kind of interesting to me to hear it argued that what's going on in the present really matters that much at all, considering Ubisoft themselves lost faith in that aspect of the series and have largely dropped it after they (very clumsily) wrapped up Desmond's story, instead of finding some way to keep the present stuff going. If the present-day stuff is really so important to the series, why have Ubisoft have decreased its importance in the games themselves and increasingly focused on the past stuff? In the latest AC games, it's barely more than a few winks and nudges here and there to remind you that the present exists, but if it really was that important to the series, wouldn't Ubisoft have found a way to dedicate more time to it than that even post-Desmond, to make sure people understand it and how important it is if it truly is that important to the series, instead of more-or-less letting the past segments stand entirely on their own at this point?
This just doesn't seem like an honest or consistent argument to me when it doesn't it seem to be held by the game's creators at this point who seem to have taken the fan feedback into account by more or less excising the present segments which, wile it can be interpreted in a number of ways, definitely doesn't speak to the confidence they had in those segments or some vitality to understanding the past-segments or the inability of the past-segments to stand on their own.