If you understood it, you'd see it.
If you had no extra air space with a higher oil level, then yes it would stiffen it up...but you have added airspace wich allows for the softer feel during short strokes, as if you had a lower level. When the suspension is stroked hard and fast, the restriction at the air flow controls keeps the air from moving into the chambers quickly, and allows the higher oil level to do it's job.
Sorta the best of both worlds. It's a speed and volume thingy.
Not trying to be smart, just stating facts.
It's actually an ingenious design.