I can't believe this is actually newsworthy. Must have been a slow day at grunge world.
There could have been any number of millions of possible reasons why he said STP and like someone else said, it wasn't like he said they were bad or anything. Maybe he's actually a fan, so that band was the first one that popped to mind. What if someone said "Play some KISS" would he have then replied, "Oh, how about some Soundgarden?" causing a tailwind of hatred hurtling toward radiohead from countless Soundgarden fansites.
STP is my favorite band too guys, but I don't really think it is of any particular significance every time anything at all, positive, negative, nuetral or as I see in this case, completely meaningless pops up.