> People somehow thought this would mean checking for ad block?

No, the worry about adblock was that Google could now just remove the ability for that work in chrome and there would be nothing you could do, because your forked chromium wouldn't pass the attestation check (cause it would never be blessed by Google). This concern extends to other browsers as well. Because Google is making themselves the signing authority here, they can choose which browsers are allowed and which aren't, letting them force other browsers to add or drop certain features if they want to be granted attestation.