> And why did flu apparently disappear during COVID?
Because they stopped tracking it to give resources to tracking SARS-CoV-2, nothing more. The CDC had a note at the bottom of their site explaining why their graphs flatlined, and despite it people used those graphs as evidence it really did disappear.
It disappeared because of social distancing. Influenza is less transmissible than SARS-CoV-2, so the measures used to keep SARS-CoV-2 under control were even more effective at shutting down transmission of the flu.
Because they stopped tracking it to give resources to tracking SARS-CoV-2, nothing more. The CDC had a note at the bottom of their site explaining why their graphs flatlined, and despite it people used those graphs as evidence it really did disappear.