I'm not qualified to analyse this, I read into it a bit more and apparently he was trying to complicate the game, the 1973 tournament book only marks it as ?!.
The engine knows what move would be a blunder for computer play.
It doesn't know what move made the position impossibly tricky for a human to maintain for black.
Every game that I analyze, there's possible moves where the computer will trade endless complexity for one's side for a couple centipawn advantage. These are moves that a human should not play.
I think the main point is that computers are so much better than humans at playing chess that what might be a reasonable move for them is effectively a terrible blunder for a human. It's like comparing a student driver with an F1 pro, and saying, "It's totally reasonable for the student driver to yank the wheel to the right when driving at 300 km/h, because an F1 driver would be able to recover from that." The point is that the student driver won't be able to follow up with near-superhuman ability to save the situation.
Of course, in this case, the engine doesn't think this is a particularly reasonable move; the evaluation just fails to show how bad it would be for a human-- even a near-superhuman chess player like prime Fischer-- to play.