The 15 Hr Wonder you bemoan had to pass the same CFI-G Checkride using the same PTS Standards that any other CFI-G Applicant has to pass. The point that low time CFIs are Instructing low time students is valid but has been the reality in power instruction for decades. If it was creating large problems it would have manifested itself long ago. In the Power world, Instructing is generally a way to build flight time after which the pilot moves on to less stressful, more lucrative assignments.

For me, the CFI Initial Checkride (1990) required more intense preparation than for my ATP (1993) and later Type Ratings on a jet airliners. As a very recent CFI-G Applicant (2019), I approached it with the same level of preparation as my initial so long ago. Do I know everything about gliders? No. Do you?

As Tony pointed out, the 15 hr. requirement is PIC, which means glider time AFTER certification as a Private or Commercial Glider pilot. Unless this low glider time pilot has access to a modern high performance ship, they may have had a lot of flights to get that 15 hrs.