But the vast majority of samples are coming from people who will ultimately survive, but who have respiratory illness. Anyhow, scientists have considered testing bias.

https://www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6937a6-H.pdf

“Initially, declines in influenza virus activity were attributed to decreased testing, because persons with respiratory symptoms were often preferentially referred for SARS-CoV-2 assessment and testing. However, renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus.”

Most people don't die from flu, so that means it might be underrepresented by 0.1%.

If anything, there's less testing for flu because of covid. But nonetheless, this is sampling, not testing, the WHO performs consistent randomized sampling over time to keep the data meaningful. If they just were based on testing of reported flu cases the data would be heavily biased and basically useless in tracking the disease.