Anything like bitcoin requires both sides of the transaction to be online. You couldn't do that over interstellar distances via a disconnected service like the X boats. You couldn't run a service like SWIFTT or BACS over Xboats either. There is too much opportunity for a man-in-the-middle attack as you can't do an on-line verification of the source of a transaction. All you get is a message turning up out of the blue and you have to rely on the message header to tell the truth about its origin. You can't interactively query a certificate authority to validate the identity or signature of the message.
Interstellar funds transfer is going to rely on something like bearer bonds, bank drafts or good old cash that is designed to be hard to forge. You couldn't run a secure banking network over the Xboat service.
That’s not actually the way certificate authorities work. They sign a vendor’s key and clients have copies of those signing keys so they can verify the signature of a key a vendor gives them. Your OS (or browser in the case of Firefox or Chrome) ships with a bunch of signing keys for certificate authorities. When presented with a TLS certificate the browser can verify the chain of signatures back to a certificate authority. There’s no connection that needs to be made back to the CA to do this verification.
Offline verification is just verifying a key provided by someone is signed by some third entity that you trust (or de jure trust like a government seal). It’s trivial to keep a large list of current and historical signing keys for governments and certificate authorities everywhere. It is a standard feature of cryptographic libraries today and would likely still be so in the Far Future.
In the case of bitcoin there doesn’t necessarily need to be an active or persistent network connection to operate. Bitcoin is just a distributed ledger. A bunch of transactions can be made before they are actually verified by the mining network. It would be possible (though not without challenges) to run bitcoin over a sneakernet effectively.
In the 3I one of the functions of the Imperial government would be to provide signing keys for sector governments (and they provide keys for planetary governments) so people can verify the chain of keys and know some vendor has a valid Imperial charter or license to operate. This chain of trust can also be used to verify government communications. Cryptographic signatures would be the basis of much of the operation of the Imperium. Nobles would pass down their cryptographic keyrings to heirs instead of signet rings.