But, I'm slightly puzzled. All instructors that I know received a letter
from the FAA/TSA about a month ago explaining these requirements. If Slick
is a US flight instructor, why didn't he get that letter?

Tony V CFIG

Maybe he did? but IIRC the letter did not specify the exemption for Glider
students..
so he took the training and is trying to follow the rules?
I do not believe the training specifies the exemption.

BT CFIG