That's not correct. Vaccines often don't provide perfect immunity against diseases but reduce the effect to a negligible level. The smallpox Vaccine is a famous example. This is why the scientists are stressing that we don't know if the vaccines will prevent people getting minor infections and passing the virus on.
The protection level that's being referred to is a simple ' someone caught it and we detected it' level. If AZ were trying harder to detect infections then the protection levels aren't directly comparable.
Also over a huge scale deployment you should be able to tell if the infections are milder even for those vaccinated but not fully immune. We don't have any good data on that yet for any Covid-19 Vaccine.
Edit: is there a chance we're talking past each other but all agree here?