5 Seconds of Summer is actually listed as one of the "influenced by Green Day" bands on their Rock and Roll Hall of Fame exhibit. They're probably the most embarrassing one, but still, I'm assuming they pre-approved a lot of what was included on it. Maybe? I don't know. (Edit: Wait, probably not, because it says Tré was the drummer for their first album, heh. ) Honestly, to me, it doesn't seem that farfetched at all for them to reach out. And even if BJ did decline and say they were "too busy," maybe he was just being polite. That doesn't mean he would've necessarily did it if he wasn't busy... it was just a good, timely excuse. Nothing wrong with that. I mean, he did write with Avicii recently, and the Lonely Island, and Norah Jones... and lets be honest, all of those pairings sound just as absurd as writing with 5SOS, if not less.
You can't argue that all of those bands were in some way inspired by Green Day, though.