I don't think it necessarily think standalone means we have to be picking up with them at the start of something where we're ahead of them. Standalones can pick up the situation on the run so we can avoid the dull part where the characters catch up to us.
Agreed. While I think there's still creative ways you can continue with the protagonists being less informed than the audience, this path can absolutely work too, while still informing new audience what's going on. Colonial Marines are in firefight. Booms
aplenty. Weird spider-hand-like creature jumps on one of the Marines' faces. New audience member wonders what's up with that? Marine yells "HE'S BEING IMPREGNATED!!" Eww, okay I get it. A simplistic example of using brief explanations through dialogue without bogging down with life cycle explanations that fans know it a million times over.