Thanx to all the posters who have already tested out what I was planning to.
The 'captured with low health' seems to be identical to the Civ VI Mongolian mechanic of capturing enemy cavalry units: by the time you defeated them to capture them, they had very low health left, and frequently did not survive another turn if any enemy units were left in the area.
Capturing enemy Treasure Fleets was already hinted at in one of the early pre-release Blogs, I believe. But of course, nobody could be sure if the mechanic actually worked the way it was supposed to, as wonky as the entire released game turned out to be.
Will note that with about 250 hours in the game now, I have Never seen either any Corsairs used by the AI, nor any Treasure Fleets even attacked by the AI, let alone captured - another Human Only mechanic, it seems.
That will not stop me, in my next Exploration Age game, from hoisting the Black Flag and setting forth in a Carrack named the Jolly Roget (the only pirate ship in history manned entirely by English majors) or the Black Pearl Jam (the only Grunge pirates) and committing inflicticide on enemy Treasure Fleets. Assuming, of course, that the AI Civs bother to form any . . .