Dana White is a drama promoter for the UFC like Vince McMahon for the WWE. He will make up BS at the drop of a hat for the attention.
I'm sure the truth is somewhere in the middle. Aldean probably did tell him no, I wouldn't be surprised if his reason was he didn't want to perform in Vegas, but logcaly speaking, from a performer standpoint, SNL has about 100x the audience and 1000x the staying power over the UFC.