Docker's CEO is being disingenuous. When you deploy a Docker container, you specify the image ID. The ID looks like a SHA-256 digest and even starts with the string 'sha256' but it is an arbitrary value generated by the docker daemon on the local machine. The ID is not a hash of the image contents [0, 1]. In other words, docker images are not content-addressed.
Since docker images are not content-addressed, your image registry and image transfer tools can subvert the security of your production systems. The fix is straightforward: make an image ID be the SHA-256 digest of the image contents, which is the same everywhere: on your build system, image registry, test system, and production hosts. This fix will increase supply chain security for all Docker users. It is massive low-hanging fruit.
Now Docker will add image signatures without first making images content-addressed. Their decision makes sense only if their goal is to make money and not make a secure product. I cannot trust a company with such priorities.
[0] https://github.com/moby/moby/issues/39247#issuecomment-49697...
[1] https://github.com/distribution/distribution/issues/1662
EDIT: Added another link.