Thanks, Walt.
With transponder and on ATC frequency you will hear other pilots being warned of your position relative to theirs, just as the 421 in this case. Typically "traffic, a glider, (distance, relative direction, altitude), maneuvering". Personally haven't heard detail on climb, whether rate or just "climbing".
Flying with another glider, each using discrete codes ATC has not issued maneuvering instructions in my experience. Perhaps just "do you see the other glider?"
The controller would assume an aicraft squawking VFR was on another frequency.
Even before the 1201 then 1202 VFR codes, controllers could tell a glider squawking 1200 from other aircraft. A fiberglass glider without transponder could be invisible to radar.
If something like this happens to you, it's possible to submit a NASA form which is supposed to be anonymous and used to help prevent future occurrences.
Jim