The reason for the minimum length is not insufficient GPS signal. It’s to meet the spec on rejection of interfering signals on certain frequencies. You won’t know if your short cable was the best thing to do until or if you encounter the max interfering signal that the box is still supposed to function with, and it doesn’t work. And without sophisticated instruments the only real test is if you immediately plug in longer coax and it solves the issue of not working.
The same thing goes with hidden gps antennas. Unlike ILS signals, which are highly standardized, GPS signals vary greatly in strength. And they’re weak to start with! You never know how good or not your hidden gps antenna is until you fly into truly marginal reception conditions, and your gps drops out while the guy you’re in formation with-with an external antenna-continues to receive his gps data.