I will say I'm surprised it has taken this long to officially make it to rumor status...
"Within THE SECOND GATE, the Marvel Characters will be primarily utilized as part of THE MARVEL UNIVERSE, although they may also be used throughout THE SECOND GATE as strollers or featured elements in stores, restaurants, and the like (subject to Marvel’s reasonable approval). Within THE MARVEL UNIVERSE, the use of any non-Marvel characters will be subject to Marvel’s approval."
This is (what I see as) the most relevant part of the contract. Specifically the last sentence. No way Marvel would allow outside characters (plus who knows what kind of contracts there may be between Marvel and DC)
There's some more nuances in there (as to where The Marvel Universe exists, and how much is allowed to be in other areas) but any chance of mixed comic properties is non-existent