Could you explain why this is "literally so stupid" when this appears to be what is happening?

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.