At the time when the original research was being done, authorities were a lot more diligent about testing but it's safe to say they really don't know how many people had it. They also don't know how many people have long Covid, nd a Covid related heart attack or stroke will almost certainly not be recognized as such unless the patient is being treated already.

In this particular study, less than half the people in the Covid group were sick enough to be hospitalized.

They couldn't measure vaxxed people if they wanted to go back as far as they did because the vaccines didn't exist. The subjects all had a pre-Omicron strain as well, which throws more uncertainty into it.

The ball to keep your eye on is the excess mortality rate, which is still way higher than it should b3, even though deaths related directly to Covid are way down.