The part about sharing data with "a very few carefully vetted security researchers" doesn't preclude them from leaking domains. For instance if the security researcher exports a "SELECT COUNT(*) GROUP BY hostname" query that would arguably count as "summary form", and would include any secret hostnames.

>https://quad9.net/privacy/policy/#22-data-collected

If you're trying to imply that they can't possibly be leaking hostnames because they don't collect hostnames, that's directly contradicted by the subsequent sections, which specifically mention that they share metrics grouped by hostname basis. Obviously they'll need to collect hostname to provide such information.