What does it add to our knowledge? That some sample of people have more antibodies than a non-equivalent sample? I could have told you that without a scientific study.
That same sample of people who had the illness may have had more, fewer or equal antibodies if they had not gotten sick and instead got vaccinated. This study does not tell us anything about which is the more likely possibility.
No you couldn't. You could have guessed but you wouldn't know. Now we have numbers. We know the level of immune response and the distribution. Like I said, imagine the opposite. If convalescence produced a weaker response than vaccines. That's equally possible and would have lead to some serious policy shift. Now we have a clear answer and can make informed decisions.
I couldn’t have guessed that we could find two samples of people such that antibodies in one group exceeded antibodies in the other? That is true for any non-uniform data set.
As for your other point, I also maintain that the opposite finding would have told us equally little. Perhaps they had lower antibodies because they had less virulent variants/cases on average, which is why none of them died - again, “survivor bias”. It would still tell us nothing about how their immune response would compare between the case where they got sick and the case where they got vaccinated.
Edit: here is a simple challenge that can resolve this discussion in your favor. If you can tell me definitively what the expected antibody level for the survivor dataset was prior to this study, I will concede that you are right and this is a meaningful analysis.
I'm confused. If I could tell you want the expected level was before the study, then you'd be right not me. My point is that we didn't know for certain until the study was done. We could reasonably predict it would be "some" but not that if it was more or less than a vaccine response.
Scientists can’t develop a null hypothesis about the impact of the most studied virus in the history of humanity? If you have no null hypothesis then you shouldn’t be drawing conclusions from data, period.
Anyone who reads this site regularly is probably aware of the problems with non-registered data analyses in academia (often referred to as the “replication crisis”).
We should be incredibly skeptical of any non-registered analysis, even when sample controls are used. When the study is unregistered AND the sample is uncontrolled the default posture should way beyond skepticism - it should be considered meaningless until reproduced by a better-structured analysis.
That same sample of people who had the illness may have had more, fewer or equal antibodies if they had not gotten sick and instead got vaccinated. This study does not tell us anything about which is the more likely possibility.