Hopefully, they will do just what Brave did - embed an ad blocker compatible with uBlock lists right into the browser. This way, Manifest V3 might blow up in Google's face if a sufficient number of browsers come with ad blockers out of the box.

That being said, they are not talking about Manifest V3 extensions. It's just that they can't test all the extensions and will have to work out the issues as they implement the extension API for mobile.