As Loki stated
here on reddit, there was nothing to sell out. Both teams appreciate the game created by the other team, and a collaborative project was created out of each team's shared interest in the other. Hard to say anything is a sell out when both updates are free for players of the respective games, and literally does nothing but bring more content to both games.
Obviously, it would be nearly impossible to explain what all of this stuff was if we didn't at least say "Oh hey, this stuff is from Dungeon Defenders 2!" Its no more "advertisement" than we or they do for each update that is released anyway. Just this time, it mentions the other party.