That I've seen, the problem is worse than that. A movie merely says it's "based on a true story". If you're a lawyer or literature professor, that "based on" might be correct usage - since 40-ish percent of what the movie told was true. The other 60-ish percent was utter fiction.
Meanwhile, people who saw the movie and found it decently engaging are busy convincing themselves that it was 99% true. And 99% of 'em will never bother to check.
Actually 99% do understand it’s not all factual and do bother to check.
There is no data to support your last paragraph. But it is fun to talk about how dumb the “other” people are.
I coulda added another "That I've seen" disclaimer to my second para. My dataset is just friends & family who I've seen "based on a true story" movies with, where I happened to know the history.
The term to describe my "99%" isn't "dumb". It's "don't care".