I would considered alt history a sub category of historical fiction. They are all a fictitious story using historical settings. Alt history movies just go further and change major events as well, when inventing the fictitious story they wish to tell.
If anything I would lump movies like Gladiator and Braveheart in with Inglorious Bastards as alternative history, because all those movies really care nothing about historical accuracy outside of loosely using it as a setting to make their own characters that have little to do with real life characters or events.
Even my favorite "historical" TV shows like Rome and Wolf Hall are still historical fiction because they are changing the events that happened, even though they are much closer to what happened historically and as such, it's important to label them as fiction.
There are few movies I would label as just straight historical movies, without any sub labels, where they try to stick to historical events and characters, as they happened. Asshole Hollywood types have corrupted the moniker of "based on real events" type movies, but it's what I think of when the most fictitious part of the movie is the dialog and minor changes to character interactions. Gettysburg is by far my favorite of these types of movies (and movies in general), but it would also include movies like Midway (1976) and Dunkirk, not to mention TV shows like Band of Brothers.