As soon as the media asked the question and the UFC formally stated that they were interested in making the fight, even saying that it could be held in Dallas Cowboys Stadium, an offer was made.
How did Mayweather vs McGregor happen? It didn't start through a "formal offer." The fight happened because everybody wanted to make it happen, and it was all based on a rumor and a photoshop of McGregor facing-off with Mayweather.
Again, Anderson Silva wanted the fight. The UFC wanted the fight. GSP is the only party who didn't push for the fight. So, the fight wasn't made, and now we are stuck with pointless hypothetical debates involving MMAth. LOL
This is why I fully supported Mayweather vs McGregor, and I continue to support fighters who make reasonable call outs. The truth can only be known after it happens.