The 20-minute protocol requires the 5-minute max effort to reduce the anaerobic contribution to your 20-minute effort. It’s possible you have a very strong anaerobic capacity relative to your aerobic capacity, which could lead to ramp test results that might not be sustainable given your aerobic fitness and capabilities. It sounds like you could probably train with the FTP given from the ramp test, but that you might dial down the intensity of some of those workouts and work to develop your capabilities at sweet spot and threshold as those may be weaknesses (and possibly limiters depending on the type of racing you do).
Short answer: yes, the 5 minute effort should be maximum effort. I’ve set 5-minute power records on several occasions during the 5-minute precursor to the 20-minute FTP test efforts on the road.