> There’s a vulnerability in Signal where you can set up linked devices that replicate your signal messages.

You mean the desktop linking feature? If that's considered a vulnerability, then so is being able to chat with someone after getting their public key unverified from an overseas server, the primary mode in which everyone uses it (including the people in this chat, evidently, since no out-of-band key exchange was performed)...

Not to mention the "vulnerability" where you copy the phone's storage and get the key material onto another device to do with what you will, which may be harder or easier depending on the hardware but I'd trust any sufficiently funded security agency to be able to do this for common devices