The purpose of CT is so that shenanigans have to be done out in the public view where they can be more easily detected. Depending on the logs you are using, an attacker may not be able to submit a self-signed certificate (I haven't looked at which logs Chrome is using these days).

Regardless of the documentation, Chrome does roll out changes to protect against local threats. I think they just don't want to be on the hook to address every local threat. Happy to give examples in private if you want to email me.

Disclosure: ex-Google employee