As for during the writer's strike, I believe they could still film what was written. They just couldn't write more or change the script. So I;d guess the idea might have been to film as much as they could, then do rewrites and reshoots after the strikes end.
Deadpool 3 is filming using a shooting script written by WGA members. Barrymore and Maher would have started filming with themselves replacing the work their writers would've done.