Perhaps in the RN medical assistant is not considered a specialist submarine role, since medical assistants serve on surface ships and shore establishments. Prior to being posted to a submarine they undertake specialist courses, in radiation, atmosphere monitoring, etc. However, perhaps even those qualified for submarines are still seen as belonging to a core trade that is RN wide, but is undergoing shrinkage with the RN in general.
A reasonable point but I believe the ability both physically and mentally to be able to stay underwater for an extended period of time is way different from someone serving on a surface ship or onshore.
Its a cack handed way of dealing with people.