I assume hes been on lots of movies with guns. In my experience, any decent firearms person trains someone that you treat every weapon as if it is loaded, even if it isn’t.
It goes back a bit longer than that. Baldwin's father was a Marine, an expert rifleman and a high school riflery coach.
It is his fault. Anyone with an IQ of over 3 checks a weapon personally.
It’s rumored that that is the briefing that the know-it-all producer/actor refused to attend.