Yeah, that's mostly a BS excuse, because movies take much longer to make than TV series, so while it'd indeed be difficult for the movies to feature TV characters, it really shouldn't be that hard for, say, AoS to work in movie character cameos... as evidenced by the fact they did just that several times.
in this particular case, however, that excuse doesn't wash at all. Infinity War/Endgame were filmed back-to-back two years ago, meaning that the major story beats, if not entirely finalized scripts, would have been locked down even before that. The AoS S6/7 renewal, OTOH, was made a year ago. So, the AoS showrunners knew from the very start of their writing process that the Snap wouldn't be simply undone, and that there'd be a five-year time jump by the end of Endgame. Instead of laying off half the cast and diving into the details of the post-Snap world, then, they made a clear choice to diverge entirely. It's an understandable decision, especially given the TV/Movies behind-the-scenes divergence, but it's got nothing to do with production schedules.