Yes there is such evidence
http://www.mortality.net/modules.php?op=modload&name=Downloads&file=index&req=getit&lid=102
The above link is Erik's interview from last October on Mortality radio (good interview too).
In it Erik makes it abundantly clear that both
Dungeon and
Dragon's readers want D&D and only D&D, to the point where even the presence of D20 and Star Wars simply will not be allowed in the pages of
Dungeon or Dragon.
I appreciate this point of view and presume that it is solidly backed up by reader surveys.
But it goes beyond that. Even if it is D&D based, if it is an article based on a setting published by a D20 publisher, Erik says "that is not likely to happen in the near future". (See above show at 1:23:45~1:24:45)
Praphrasing Erik now: "This is not based on a contractual commitment, it is is based on feedack that Paizo has received from its readers."
It's hard to dispute that - as I wanted adventures in
Dungeon to be D&D and I was not a fan of Polyhedron "stealing" D&D adventure space.
FWIW, At the same time, perhaps perversely, I would be very pleased to see other D&D and even D20 settting articles and product information in
Dragon. That would make me want to buy the magazine just to get that exposure as the current article mix is not to my taste.
So perhpas it's just not as cut and dried as it appears.