It’s both. In episode 1 someone asks the cowboy actor to “do the thumbs up”. Later, the actor explains the mushroom safety part to his daughter. In episode 3 there is another flashback where the cowboy actor does a photoshoot for Vault-Tec. He asks the photographer “what if I try a thumbs up?”
I’ve seen the Charlton Heston one, and the 2001 Mark Wahlberg one. The original was obviously better. I hadn’t seen any of the current cycle until I saw Kingdom last weekend. It was fine.
I listened to a recap of the previous 3 films and it didn’t matter. Kingdom takes place “many generations” after the third movie so all the other characters are dead. Kingdom’s story works fine as a standalone film. It’s not amazing, but there’s nothing particularly awful about it. Now I’ll probably see the next couple, but they’re not high on my list.