I don't think this is a problem. In the email vs OTR debate, signed emails are not forgeable because you are not supposed to give away your private signing key - to claim that someone forged a signed email, you must convince that your private signing key was compromised at that time.
However, in this case you don't hold the private signing key, so Google can make whatever signatures it wants, even of things you didn't say, and there is no cryptography that links it back to you - because as a Google chat user, you don't have a private signing key.
That makes sense if there is a dispute between you and Google. But if the dispute is between you and one of your contacts instead, to claim that the signature is forged would be to claim that your contact has Google's cooperation. That bar might not be as high as claiming your private key was compromised, but it is still quite high.
Ah now careful there. Its a secret algorithm so we have no idea how crypto-secure it is. It appears to have a constant salt and not too many inputs, because his testing showed the same output with similar inputs, so I wouldn't expect much. Even a dumb cryptographer would include a random 8 bit salt so you'd require an average of 128 cycles before noticing a duplicate, so I don't think its intentional crypto, although they'd know that I/we'd know so they'd know to ... this turns into annoying paranoia.
Possibly the contact has formal written GOOG corporate cooperation. But the theoretical minimum to know the cruddy secret algo is extremely low, like someone who knows someone who used to work there, or obtained the disk image of a stolen or improperly disposed of GOOG laptop or server hard disk, or someone who was bribed or was acting as the agent of a national government while being employed without GOOGs knowledge. Shoulder surfing an employee at the coffee shop, overheard something, etc.
Thats the problem with a cruddy crypto algo. Its a cruddy crypto algo.
Now what it probably is, is some kind of verification toy to prove internally some translator / load balancer thingy didn't mess up, or something probably very innocent like that. Of course if they were actually rolling out something evil thats exactly the right way to present it. Hmm.
However, in this case you don't hold the private signing key, so Google can make whatever signatures it wants, even of things you didn't say, and there is no cryptography that links it back to you - because as a Google chat user, you don't have a private signing key.