Would this not defeat the purpose of responsible disclosure? As a bad actor I could learn of secret vulnerabilities from this channel.

These patches are available to all vendors who chose not to protect their users yet.

Releasing binary patches is allowed, this is why GOS have added the security preview channel.

You have google to blame. GrapheneOS tried very hard to make sure they have those security patches as google delays publishing the source tree and it's only available to OEMs