How does signing a message make it any less (or more) ephemeral? You either store the copy or you don't (and Google does). I don't see how a signature could influence that.
Whether Google stores your message when you turn on "Off-the-Record" is of course something we can never know. But if your contact says you said "X", and you say you never said "X", Google could still prove you said "X" at some point. That does not fit the idea of "Off-the-Record".
The article and/or its interpretation is wrong about that because the algo is unknown.
Even worse, human beings both currently and formerly employed by GOOG know the secret-ish algo, and those humans are not necessarily still employed there, nor are they incapable of communication. So its very unclear who knows how that algo works other than GOOG as an absolute minimum.
A correct statement would be, at the minimum at least GOOG knows how to make those hashes, and at this instant who knows how many other people or orgs who may or may not be in support or opposition to you.