It's sort of interesting (it actually doesn't seem to be very good, with an R2 of 0.4 and 80% sensitivity at the high end), but I can pretty much guarantee from the comments here that people have no idea what it actually says. Just look at the sibling comments here -- most are about whether or not countries are under-reporting deaths.
This is just more fuel for the eternal debate about Covid being bad.
As discussed in the paper: there are multiple known causes for undercounting, and this analysis, by design, only catches one of them. I.e. the two main causes for undercounting would be insufficient counting resources and deliberate manipulation, and this method is designed to detect only deliberate manipulation. For such an analysis, you would expect neither extremely high correlation nor extremely high sensitivity.
I agree it's not the best paper, and there's lots of unrealistic assumptions being made, but I still liked to read it. If nothing else it's useful to see how these models map or don't map onto other sources of information.
I also agree though that it would be nice if these sorts of things were approached differently, more from the perspective of epidemiological modeling or something.
I agree that it seems to assume too much malice as a reason for underreporting in general though.