I have just rewatched the scene and it doesn't look like it's a part of some elaborate plan.
Seconds before the conversation from the OP's question takes place, Cage seems to hook himself as well (as the others do the same). However, the first hit at the ship blows him out of his strap (and kills Kimmel). Here, I made a clip of that part:
Edge of Tomorrow: The beginning of the final attack
When they start jumping out, Rita tells him that he's not hooked in, which is the synonym for "but you cannot jump (and will crash with the plane)". Remember that not more than a minute before that, he said "take one for the team" (as a response to "what do we do if an alpha tries to kill us?"), so his reaction to Rita's comment (shaking his head and pressing her "release" button) is exactly that: accepting his own sacrifice for the team.
Of course, he's Tom Cruise, so he luckily survives the fall and (spoiler! ;-)) saves the day, but at that moment with Rita, Cage knows his chances are slim, and he just wants everyone else to go on and win the war.