I don't mean it's a nonsense that I sign the logbook, I mean it's a nonsense that I can complete the process for a LAPL holder, but not a PPL with SEP rating...
There is no "process" for an LAPL Holder.
Carrying out an Instructional Flight with either an LAPL Holder or a PPL SEP Holder is old hat. They are the same - they are both Instructional Flights. So an Instructor may (should?) sign the Holder's Logbook.
Carrying out a Licensing Administrative Action - That is
new for Instructors.
make the privileges exactly the same as that for examiners.
But Instructors are mere mortals, whereas Examiners have
Magical Powers that enable them to ascertain immediately if a Dual Flight logged six months ago was indeed flown with the holder of a valid EASA Instructor Rating.