Note that he says "often even indoors"

Compair the sensitivity spec to your Garmin GPS. Your older GPS may be less sensitive.
All GPS's work off of an RF signal from the satilites, solid objects reduce that signal.
If you get your sensitivity good enough you MAY still be able to get a fix from the wimpy signals that are making it thru the roof.

My very old GPS could not lock in if I even got near my house or even walking through the woods in the winter with no leaves on the trees. My newer unit (8 years old) can still get a lock in the thickest of woods during the summer with the leaves so thick that there is no sky visible. It will work in my house as long as I am within 5 feet of a window.
I'm sure that the newer ones are even better yet.

I also would like to know it this unit can display vertical speed or just the horizontal component of your speed. In other words will it show my terminal velocity in a vertical dive, or my actual speed in a 45 degree dive?