If lockdown measures, social distancing, mask wearing, etc. were the true cause of the massive reduction in flu cases, we would expect to also see that countries and US states that failed to enact measures would be reporting high flu numbers to the WHO.
This doesn't appear to be happening. Instead, "the drop-off in flu numbers was both swift and universal." (article quote)
This universality suggests a data problem to me rather than an environmental change.
I don’t think so. Even without government policies in place, people become much much more vary of hygiene - that alone could account for a serious drop in new flu cases. The other factor as others mentioned is that someone with a weaker immune system who would have a pretty bad case of flu, had now a much higher chance of getting covid instead (due to covid quickly overtaking the number of cases due to its higher R number), so in a way covid being more infectious, sort of suffocated the flu.
It was literally so stupid that it took me a minute or two to understand that the implication was that conspiracy theorists think flu cases were being categorized as covid.