You're quite right only a small portion of my flying is on sailplanes, although I've certainly had adequate reason over the years to understand unpowered flight, and powered sailplanes, and your quote matches my normal understanding.
The approach of full A/B, partial throttle surprised me also, but it does appear to be how some pilots operate SLMG / SSS on approach and was something that came into my pre-trials research.
Being able to use the A/B as speedbrakes is a further issue, and relies upon a degree of drag which may not exist on every design (present aircraft again, etc. etc).
G