To support which claim? That they knew naming their product "GitHub" would lead to this kind of confusion? The opposite of that is the extraordinary claim: it is painfully obvious that if you take someone else's product and name yours "ThatProductHub" you will cause semantic confusion over whether your product is directly affiliated with the other product. I am not even sure what evidence could be provided to refute that claim other than a record of extremely low IQ scores :(. People do this because they want to ride on the coat tails of the other brand, whether they want to admit it or not.
To be fair, I'll point out that you are giving no consideration to the fact that git's popularity is due in part to GitHub's massive success and that the relationship is clearly mutually beneficial.
