TL;DR: all keys are known because the list was seized after the war. However, this message was not previously decoded because the German operator mistyped the key, and also used a key from the wrong day.
This meant that ChatGPT didn't need to brute-force the entire key, just pick the correct one from the list and identify the typo.
A sufficiently dedicated human analyst could have done this; but they didn't.
If the final set of possibilities is astronomically smaller than a naive one, calling the whole process a "brute force exercise" draws attention to an astronomically insignificant part.
Legend2440 · · focus · HN ↗
This meant that ChatGPT didn't need to brute-force the entire key, just pick the correct one from the list and identify the typo.
A sufficiently dedicated human analyst could have done this; but they didn't.
msy · · focus · HN ↗
red75prime · · focus · HN ↗
rrr_oh_man · · focus · HN ↗
red75prime · · focus · HN ↗
tovej · · focus · HN ↗
red75prime · · focus · HN ↗
rrr_oh_man · · focus · HN ↗