The author seems to be arguing that it is biologically impossible or something to have a virus with the same transmission properties as COVID but higher virulence. I don't understand what principle he is invoking to make this claim.

Because fatality and virulence stand in fundamental tension with each other when it comes to epidemiology. You can't magic that tension away or just invent other incompatible properties of a hypothetical virus (stealth, latency, virulence, fatality, etc etc etc) just because you want to tell a ghost story. Biology: (immunology, cell biology, virology) impose constraints!

The cope principle