They said the key was not found by GOG. It is possible that the entire key was not pasted in the GA. If the key works for you, then try messaging them on the forums here, and cut and paste the key to them in a private message.
It is possible to change feedback by contacting support. I think there's a difference between mistakes, and scamming, but people enjoy hyperbole, so there it is.
As for a lot of people using GOG keys for GAs, yes, they do. It's probably against the rules, but rules are neither clear nor enforced.