You are spot on. They tried to pull a fast one by changing their terms of the lease without getting agreement from both parties. It may be the case that this “exception” they are making is for those who are savvy enough to have read their lease and understood what it means.
Threads like this are the reason this forum is so great. It provides so much more insight than any Motability website or leaflet. It’s even more informative than calling them sometimes.