They did in the past, but now with the new legal interpretation. They cannot. The letter is very clear in that regard.
To exercise the add on under 61.31, their certificates must be current.

I agree as a CFIG, I certify that the add-on pilot knows what he needs to know to fly the glider.
Academics and pilot skills.
But the FAA has just now stated they don't trust the instructor.

BillT