I don't understand how this will make ad blockers unusable. This new API might tell the site the user is using an ad blocker, but that tech seems to exist already..?

They can try. Maybe it gets more people to learn about pinhole or other adblocking, with a little bit of luck it even rescues one or two people from the claws of windows.

Google engineers are making my chances of using a Chromium-based browser (near) impossible

By all means... do that "Mr. Google", and I'll never touch any chromium-related browser, ever.