For influenza, the CDC includes deaths by pneumonia during flu season as part of the official death count because the certificates don't reliably list the true cause of death.
We see this happening now in the US with unofficial deaths getting lumped into the C19 deaths, some of which may very well have been flu.
Actually at least in Germany the numbers for influenza are entirely based on statistics. In particular the 25000 people died of influenza in 2017 number that is thrown around is based on looking at the usual statistics over the same period and everything above that is attributed to influenza. I think we will only have only have accurate statistics about the death rates etc in maybe a year
