I have to agree the AOPA answer doesn't make sense.

They said...

"Outside of SP is Recreational, Private, Commercial and ATP. All
powered ratings, Glider is separate. "

While there is not a Recreational and I believe no ATP Glider
certificate. There certainly are SP Private and Commercial Glider
certificates. Maybe what they meant was that certificates are separate
from aircraft Categories. Even then it is confusing to me what they
meant.

They also Said...

"A glider certificated pilot is not higher than SP. "

This is clearing wrong because a Private Glider rating is clearly
higher than a SP Glider Rating. True they did not say what kind of
glider certificated pilot.

"Higher certification means going from SP to Rec, Pvt., etc. FAR 61.31 and Subpart J
defines the differences."

This is correct, Here they say the a Private certificate is a higher
Rating than a SP certifcate.

"Taking written knowledge test to stay within powered aircraft does not required additional written test within SP. But moving outside SP to Rec or Pvt does."

This is correct but doesn't apply to the question you asked. Yes you
need a knowledge test any time you upgrade to a higher rating. (That
is what AOPA said ) According to my Test Proctor there is never a
knowledge test for a lateral or down ward move (FI is the exception).
If you have private airplane rating there is no knowledge test to move
to a Glider or Helicopter (FAR 61.63b). There is also no written if
you move downward to SP or Recreational Certificate. Knowledge tests
are only required for an upgrade in rating, not the addition of a
category.

*******************
The simple answer is that; I think it is obvious that SP Glider pilot
does not need a written or Practical exam to add SP Airplane.

A Private Glider Pilot can not add SP Glider Privledges to his
certificate because he already has these privileges as a Private
Glider Pilot. (A higher rating)

It doesn't make sense that a SP Glider pilot can transition to a SP
Airplane without the written or practical test and not allow it for a
Private Glider Pilots.

CFIIG/ASEL
Brian