Earlier quoted context omitted.
However there already exist real people in the world who have had Covid twice, and who reportedly have had a much tougher time the second time around. This study helps us put some bounds on how many people this might happen to (assuming there are no mutations to the virus) but it doesn't rule it out completely, because of the prior existence proofs
If this is rare enough, it could fall into the false positive rate of the COVID test they've previously done (i.e. while the test said they had COVID, they didn't)
You could be PCR tested positive, while experiencing a cold or flu (which you would now understandably think were Covid symptoms), but would not have immunity for a future, larger infection from Covid.
Though statistically unlikely, it's bound to happen sometimes.