Does Reed Solomon successful decoding guarantee valid payload?
No, you're missing nothing.

Quite obviously, any valid code word (sequence) should decode correctly. And since noise can lead to any potential code word (sequence) at the receiver, these "false corrects" are as likely as any other reception, if the reception was uncorrelated to the transmission.

It's just that codes are designed so that it's unlikely to happen that by pure (bad) luck you end up decoding the wrong code word when a code word was sent. But yes, it's possible, and yes, it does happen.