You can't have it both ways.
You can if air density changes with altitude, which you may not accept, but is what's being asserted.
Your own example demonstrates that terminal velocity of the same object changes with air pressure (your feather example).
Only you have just mentioned one extreme and the other... the atmosphere we experience, and a vacuum.
If the object (Human or not) is falling a distance from Earth great enough that the pressure changes so greatly from one
extreme to the other in the same fall, of course there is going to be variation in terminal velocity, and the ability for a Human
to utilise his own body to control decent is going to become more effective with the increase in air density.
I don't know the thoughts on changes in air pressure for someone who doesn't believe in "outer space",
but any citizen with the cash to spare can stick a GPS on a weather balloon full of helium and track it until it pops,
and go get it when it hits the ground. The increasing difference in pressure in & outside the balloon causes it to expand
gradually until it pops. The GPS thing... well no matter your thoughts on how that works it still works.