Man, that seems backwards to me. FTP is primarily an aerobic metric. The shorter the interval you are using the estimate your FTP, the more you are bringing anaerobic contribution into the picture (which varies so much by person that percentages are guidelines at best). The longer your “test” effort, the less the anaerobic contribution can skew the number.
As an athlete who focuses on long sustain efforts/events, I find that long intervals (multiple hours) serve a much better proxy for FTP compared to short stuff. I never have a reason in training or events to do a 5 minute or less “all out” interval. Even if I’m doing vo2max work at 5’, I’m doing multiple intervals and that makes it quite a bit lower than a 5 minute max effort.