> Thank you for helping me understand that I had missed context

My pleasure!

> The signing process retains private signing materials under the custody of the sender

In the signing process, with a symmetric key, the signing materials do not remain under the custody of the sender. Both parties need access to the signing key. If the consumer leaks the key they have to notify the sender and vice versa.

Asymmetric signing is used very sparingly in the context of webhooks.

Sorry if I'm misinterpreting your sentence a little too literally here.