The wording of the advisory isnt great, but if a car, like the Escort Cosworth has plastic sill covers, the tester is unable to evaluate the condition of the sill area around the seat mounts, seat belt mounts, because he cannot see them, so the advisory is the correct thing to do in this case.
To cover himself, He is not advising a defect, he is advising he could not complete that part of the test procedure.

The silly thing to do would be to say nothing, then find the car has a crash, somebody is killed, and its found to be because the seat belt broke free due to corrosion around the area he could not test, but he didn't tell anyone he was unable.