True, it's more of an upper-bounds of sort. There's several preprints from European researchers giving IFR's of 0.08% and 0.37%. So it works pretty well from a Fermi estimation method (https://what-if.xkcd.com/84/) (e.g. Bayesian inference really). Also, the age distribution in prison isn't necessarily the same as that of the general population. There's lots of limitations for a comparison to the general population but it gives some bounds.

I'd think the statistic of "average years of life lost" based on expected average of lifetime. Otherwise not sure of a better statistical way to measure age-adjusted IFR, which would be helpful.