First of all, that link doesn't at all support the idea that Overeem refused to fight Fedor.
Secondly, even if it did say that, which again, it doesn't, Vadim isn't a reliable source.
You didn't say which part of the statement you wanted a source for. It was obvious during the strikeforce days that there was obstacles between getting Fedor and Alistar fight done. Fedors manager isn't a reliable source when it comes to why Fedor didn't take a certain fight?