This is indeed a rather good implementation of ehat GDPR requires: clear unambiguous language, an opt-out available immediately.

This is the definition of informed consent

The GDPR doesn’t allow opt-out consent to count as consent. The only consent it recognizes as valid consent is opt-in.

However, since we are discussing the banner that The Guardian website shows to US viewers, I assume they’re trying to comply with California privacy law, which does allow opt-out regarding the sale of personal information.