Being in excellent health can increase the required pathogen dose, and help resist illness. It can also, paradoxically, cause worse results. See, for example, “cytokine storm.”

When I was sixteen I sailed a lot on the lakes in my neighborhood. I was strong and healthy and thought: what could microbes do against 1 meter 90 of me? So I drank the water from the lake to see what would happen. I became rather sick to my stomach :-) and I considered defeat

The claim was increased resilience, not perfect immunity.

about how much lakewater are we talking

Why is this downvoted? Hygiene and exercise is sound advice that any doctor'd agree with.

I think the portraits of the downvoters would fully explain this phenomenon

Probably because the poor guy's theory isn't true for all possible cases ever, isn't peer reviewed etc.

For Covid, yes. If we look at the 1918 flu pandemic, the worst affected populations were those young adults with strong immune systems. They basically went into overdrive and the bodies attempts to fight the infection caused irreparable damage to the host.