Except they’re not? Filming was shut down as soon as the SAG-AFTRA strike started.

variety.com/…/deadpool-3-stops-production-sag-str…

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.