Black anticipated ko fight with white B19, noticing that white's isn't desperate enough to play the ko, moved elsewhere for bigger points. That corner is now more predictable without the ko aji and white playing B19 now would be the same result as connecting at 1 in gote. Black 1 was thus both probe and aji-erasing move.

In essence, I agree with your guess. It's a probe so that W must announce her intention to protect the cut via ko now, before the available threats on the board diminish any further.

Trying to improve my endgame math so here's some more thinking:
W initially protecting the cut at B18 is gote 6 points up to and including the D-rank spots.

I guess I would say that this probe should be played as soon as B is confident that W will respond (at C18 or B19), and not tenuki (B's followup is only 3 pts gote, so a 9 point swing from protecting in the first place, but a 3 stone difference). So, when the value of a move is ~3 this probe can be played.

Then, let's consider after W replies at C18. Should A19 be played immediately?
Later, W has B19 for 6 points gote in the corner (still).
B has A19, which is 1 point gote. There is also a 1/3 pt followup at C19 which be left unplayed for a while.

So 4 2/3 point swing (Did I do this math right? How do we account for the unplayed 1/3 pt followup?), 2 stone difference. So when the value of a move is ~2 1/3 this sequence can be continued.

Anyone else have thoughts / corrections?