If it was Gabe Newell saying "For sure, R520", everyone would be saying that it's only ATI's money speaking. The same is true in this case.
The strong marketing relationships between ATI and Valve, or Nvidia and Epic means that we can never take what they say about these products at face value, and never know if these are true opinions or paid for advertising.
Even a refusal to voice an endorsement with a "no comment" would be taken to mean a preference one way or the other, especially in circumstances like these were there is no explantion or justification, just a statement of perference.