Well, from a Bayesian standpoint the odds of that seems to be pretty much zero, given that you would have to decipher an arbitrary message that coincidentally pointed to a real date and time that in retrospect turns out to be the correct time a boat relevant to the Germans arrived in a port.

Of course, that's given the sequence of events as written is correct, and that Astra presumably did not cheat by brute forcing all historical events around the date of the transmission in advance, found an event that could fit with the message, invent a plausible cipher to make the message fit that event, and then lie about retrospectively validating the information.

I would assume that such a process would be obvious from the reasoning chain, and so then the only remaining plausible scenario is that the writer of the article is lying.

The most likely explanation by far is that the cipher was just solved, and OP does point this out to be fair.