Isn't onchain here a transfer agent? Bearer is more appliable to cash, because it's physical. A token movement is still recorded onchain, i.e. there is a set of transfer agents and consensus among them. It can be tuned to support some regulation.
Anything controlled by a private key is a bearer instrument by definition because the person bearing the private key has custody of the asset.
You're making up your own definition of transfer agent, not the one used by the SEC. They have specific requirements and responsibilities.
At best the key signing is providing a partial audit trail with no case law to support any liability transfer created by its use. Typically transfer agents take very little liability by requiring a medallion signature before allowing a transfer.