I don't think Garmin's FTP estimate is as simple as a "best 20-minute" approximation, with the best case I can present is when doing The Sufferfest's Half Monty ramp test. Outside of the ramp itself, I was never anywhere near or above threshold and yet it came up with a relatively close (within single-digit) estimation of The Sufferfest's result. And there have been long rides where I mainly did Z2 or Z3 pacing, with not enough cumulative efforts at or over threshold to go over 20 minutes, yet at the end of the ride it would still give out a relatively close (again, single-digit close) estimation.