The lack of Flu infections in the population that did not contract COVID cannot be explained by viral interference. Viral interference would only explain a decreased influenza infection rate among those exposed to COVID.
Is it that hard to believe that not being around other people limits the chance of getting sick from other people?
These numbers seem way too low if you're talking about the prevalence of people that have been exposed to SARS-CoV-2.
It seems far simpler, in the Occam's Razor sense, to begin with the idea that staying away from people makes it harder to get sick from other people.
Unfortunately, COVID restrictions and social distancing have become so politicized that many resist the idea that staying away from people reduces the chance of getting the flu from them, because the implication is that those measures might also have been effective in reducing COVID cases. This contradicts a political point of view and therefore some would like to dismiss it out if hand.
For example I've notice that many of those who believe precautions violated their personal liberty want to diminish the effectiveness of those precautions, which isn't necessary from a logical point of view. It is not logically inconsistent to believe these precautions violated civil liberties even if they were highly effective. It just requires a person to believe that many deaths were a reasonable price to pay.