That writeup makes it clear that they're using "purge" as a synonym for "compact." I would claim that's very confusing wording, and the widespread confusion around this issue would seem to support that claim.
But version 78 has been out for many months now. So I'm curious, why you are suddenly raising this issue now?
Because, as I mentioned in my original post, I had never seen the dialogue until a few weeks ago. And, strangely enough, I haven't seen it since I made my most recent post four days ago. So I still haven't told it to do the "purge" operation.