You’re belaboring the point. Yes air is coming from below, and since the plane hangs in a nose-down attitude, then the AOA is some number below 90 and is making some force pulling forward. And maybe that’s even enough to give a measurable forward speed to the plane-canopy combo. (Like has been mentioned, some round canopies even have vents in the back that make them glide forward; but someone said that this canopy design is not one of those.)
Nevertheless, whatever forward/sideward behavior the aircraft has or doesn’t have, the point is that when descending through a uniform airmass, after any initial period of stabilization, there can be no forward/aft/lateral force form the airmass that’s any different with the airmass moving vs. not; and therefore no force that could alter the descent rate.
Separately, while I echo Pilot DAR’s call for civility and calmness in discussing even obvious points, I’m starting to sympathize (and to feel myself the same way) with those getting exasperated at posts making the same point, for the umpteenth time, without addressing what’s already been pointed out wrt. the inability for a uniform airmass to impart a force to an aircraft within it as if it’s anchored to the ground somehow.