It’s very simple.The point is that the vast number of Covid positives never end up in the numbers because they are asymptomatic and come and depart with those infected having no reason to test. Enter a virus or flu with covid like symptoms and you have all of those people rushing to test, and the tests catch the asymptomatic covid they have concurrent with the symptomatic virus or flu. So the numbers go up because you are catching more of the asymptomatic covid cases in the population. I’m not saying it accounts for all of the increase but it makes sense that it would be a factor.
It is currently estimated that around 20% of COVID patients are fully asymptomatic.* There would need to be very very many people who just happen to have been unknowingly asymptomatic within the last 2 months (who didn’t get it in the first wave) and just happen to be sick and test for COVID now and just happen to get the virus sample picked up (which, while it happens, isn’t super common) to affect the numbers greatly.
That can only possibly account for a small fraction of people testing positive, by sheer math.
*
SourceRule of thumb: When someone tests positive for COVID, they typically have COVID. There is a reason the large testing drives where they discouraged symptomatic people from testing were wildly successful.