It's very strange that Dana felt the need to address this. I think the rumor is true, with a twist.
I believe the UFC 303 PPV predictions were looking poor, and that the event was going to sell less than 1 million PPVs. In that scenario, Conor would likely make significantly less money that he thought he would, as most likely his pay was a heavy cut of PPV buys above a certain threshold.
I think Conor realized this and demanded that the UFC pay him his full expected income, regardless of how well the event sold. Naturally, the UFC aren't going to give him the same $$ if the event sells 800k buys rather than 2 million buys, so they refused Conor's demand. That's my guess.