Yes, for example your key could be stolen through electromagnetic eavesdropping (Cryptography Research had an amazing demo of a practical side-channel attack at

Yes, for example your key could be stolen through electromagnetic eavesdropping (Cryptography Research had an amazing demo of a practical side-channel attack at the RSA conference one time), but that doesn't mean that all of your message contents are exposed that way. Or, malware on your phone could exfiltrate your key but not all of your messages because that would somehow be more suspicious/noticeable, like if you were on a metered mobile data connection. But if the attacker is a government or has otherwise compromised telco infrastructure, the attacker might already have copies of your old encrypted traffic -- which, without forward secrecy, it could use. I think the context of the claim in the article is pretty much only for the "government physically seizes your phone and examines its contents" scenario: but even for that scenario, a user might choose to selectively delete specific messages without deleting all of them (e.g. Signal allows you to delete an entire "conversation" with a specific person). So in this threat model, either way, we also need to look into whether Signal successfully overwrites the content and metadata of the messages it deletes on the mobile device storage, because the government attacker would use forensic tools to try to undelete that data. I don't know the answer to that.