It’s a while since I looked, but it’s because they suggest you do a 3 minute test IIRC and it suggest zones from that. It’s not based on a ‘conventional’ FTP, but there’s nothing about the 95% thing.
From the Wattbike Website:
20 minute threshold test
What is it: This is a test for experienced cyclists only. It is usually conducted at 75-80% of maximum minute power (Z5 heart rate and power training zone) and measures improvement in sustaining a high percentage of maximal aerobic power. The 20 minute test can be used to estimate functional threshold power (FTP)
When to use it: The 20 minute threshold test is a progress test which can be used by experienced cyclists to check progress throughout a specific training plan or at the beginning of a programme to estimate FTP.
Doesn’t explicitly say 95% anywhere.