Any idea?
Yes - it's utter bolleaux. The only requirement is that the medical certificate must be appropriate for the level of the licence if all licence privileges are to be exercised.
No PPL holder needs a Class 1 medical. Also the right to be remunerated for conducting flight instruction is a PPL instructor privilege and has nothing to do with the medical whatsoever.
Whoever is peddling rumours to the contrary needs to STFU and to RTFM - in this case EASA's Easy Access Rules for Aircrew.