He's a board member and part of the negotiating board for the contract, so he's as official as it gets. It also helps his image that a large part of his career so far has been specifically about deconstructing historical propaganda or popular myths, so it fits like a glove. Other than that, they also have other very outspoken people in other unions like Chris Smalls and this Shawn Fain guy isn't particularly charismatic but at least he doesn't hold back.

It's actually a bit strange how SAG-AFTRA doesn't have some big charismatic actor doing the representation AFAIK, but better that than having another Ronald Reagan.