1) I'm not American
2) Many Deaf people know multiple sign languages, just like many hearing people know multiple spoken languages. This might be more common outside the USA. I know several languages both spoken & signed to varying levels. (reading / writing only for the spoken ones.)
3) The actors only started learning signing a few months previously. It's impossible to become fluent in that amount of time. In the signing in the film the eye contact, accents, grammar, emphasis, role-shifting etc all have issues that are noticeable to fluent signers.
I wouldn't know but I'm told that for hearing people, you can tell if another person isn't fluent when speaking - even if you don't know their language well - because they have issues with their word formation, stumble over pronunciation, miss out letters, put pauses and emphasis in the wrong places and so on. To me as a deaf person they would look like a fluent speaker, but you as a hearing person probably would hear the issues in their speech very quickly.
Bonus factoid: ASL is quite similar to LSF for historical reasons - they only started diverging a couple hundred years ago.