I wrote the question below based on what I saw at the end of the game. But going back to the game, I now see a different AI analysis that makes better sense. Or does it? Am I going crazy?
In game Partie amicale, the AI analysis says that white made a mistake at white’s move J2 (I can’t read the move number from the tiny move chart). It says that white should have moved to H7 instead, continuing with black H8, w J4, which secures life for the w group.
But it looks to me that after those moves, b H3 would kill the group for sure. How would w respond to b H3 to save the group for w?
No. J4 is very useful here, it means black can’t atari from j3 due to liberty shortage. This hanging connection on 1st line shape is a common eye building tesuji. Like j2 it defends h2, but indirectly and with fewer points but crucially more eyeshape.