"What makes this impressive isn't actually the codebreaking, but that Astra completed the entire multi-modal workflow in ~6 hours from a single image and goal."
It’s an image of ink-on-paper cursive handwriting, with what looks like markings added during the original decryption to keep track of where they were in the process.
So image to text, then a brute force attack on a Napoleonic era cypher.
https://carter.church/writeups/the-letter-to-marmont/