Exactly. See Sidney Dekker's Field Guide to Understanding Human Error. The posters do (almost) nothing -- other than covering the backside of those who put them up, and doing safety theatre. Looks good, does nothing for safety.

The same author, Sidney Dekker, has a very good book about how to deal with someone like this whistleblower. It's called "Just Culture". Well worth a read. Spolier: It's not to silence them, not this way. You can silence the person by, you know, actually doing something useful. But that requires actual change, and more importantly, a change in attitude. And you need to convince the crowds that you are doing better by NOT putting up posters. You'll be surprised how many people really think the posters help.