If you follow the letter of the regulations as they are written, there are so many things that nobody can instruct for, because it is mentioned nowhere in the instructor privileges.
For example, there is no paragraph saying that an FI or CRI can do familiarisation or differences training...
So you can either argue that nobody can give that training, file reports whenever you see somebody doing that sort of illegal instruction, and refuse to accept any differences sign-offs issued since EASA part FCL came into force, or you apply some common sense and carry on as normal.