Blocking third party cookies is great, but with these "Third-Party Sets", self-appointed gatekeeper Judge Google (Judge Dread) takes requests from website owners for lists of domains they control to be partially exempted from these third party cookie restrictions.
Instead of making third-party cookies obsolete as is one of the exclaimed goals of the broader initiative, they turned it into an insidious and opaque tool that lets them increase their insight, influence, and control over the web and all its users even further.
This is bad for both advertisers and their prey alike.
Addendum: Getting rid of third-party cookies whilst not breaking Single Sign-On and similar such features could also have been achieved with a user-controlled local browser setting and a new type of permission request popup.
The need for Google to be in control of this is non-existent and the people working there are more than smart enough to understand this. They are playing us all here.