But whatever you use would still be easier to implement using traditional tech than web3, because the problems they solve are orthogonal.

Yes. I regret not being more clear in my original comment, because the scheme I alluded to is an application of public key cryptography, such as Certificate Authorities, and is not about cryptocurrencies specifically.

Even with time stamped signatures you can clone the signature onto a fake watch.

The point of a signed timestamp (and/or challenge string) is that it demonstrates the signature is freshly generated, proof that the device is authentic right then and there. An old copied signature would not have this property.