> On the other hand, if you tell me that something is a "REST API", I already have a very good idea about how to use it (...)

Precisely this. The value of words is that they help communicate concepts. REST API or even RESTful API conveys a precise idea. To help keep pedantry in check, Richardson's maturity model provides value.

Everyone manages to work with this. Not those who feel the need to attack people with blanket accusations of mediocrity, though. They hold onto meaningless details.