Moving Flat Out is not a declared action, like ramming. Flat Out is just a third level of speed. INAT describes a situation in which the Ork vehicle is simultaneously moving at two speeds: Combat speed for the purposes of the Orks shooting out of it, and Cruising speed for the purposes of people assaulting it. This is the exact same situation: the Dakka Jet moves Cruising speed for the purposes of shooting, but Flat Out for the purposes of incoming fire.
To clarify: the 4+ cover save is granted to any skimmer that moved Flat Out in its last turn. Flat Out is defined (on page 70) as moving more than 12". Since inches moved is the only arbitrator of the level of speed, that;s what we have to go by. Ridiculous? Yes. Legal? Also yes.