I think the point is that when a new movie is made, many people put them together so that one informs the other, in the same way that the first half of a movie informs the second half.
it’s relatively easy to imagine a movie that starts great, but the second half introduces a major twist that makes the first half seem dumb.
For sequels/prequels we have the ability to just say “Well that one isn’t canon”, in a way we can’t for a self contained movie. But some people (including me) have a hard time with that, because it seems so arbitrary. The world is no longer something I am experiencing and enjoying, now it is something I am controlling and curating, which hurts immersion.