"Astra's hypothesis for why this particular message was previously unsolved is that “TRUPPENVERSCHIEBUNG” was used as the key starting on December 9, 1918 - whereas, as noted above, this message was transmitted earlier, on November 27, 1918. The reason for this discrepancy is unknown."

So it used a known key. It didn't come up with a key from thin air. The only gotcha is that apparently this key was used two weeks earlier than it was documented (maybe the operator was using the wrong page from the codebook?).

Creating a fake key that decodes the original message into a valid result (including matching the ship's arrival time to the day) would be significantly more impressive than just cracking it.

From the article:

> Astra felt compelled to check its work and found that, in fact, the English cruiser HMS Canterbury arrived in Sevastopol on November 24, 1918, based on its original logs

Then follows a picture of the original log papers.

I don't think the described cipher has enough degrees of freedom for that to be possible.

That’s statistically unlikely (to not say impossible), isn’t it? Plus the compute required to brute force a key is not available at inference time.