Well, it's worth keeping in mind that the risk of death from a coronavirus infection is comparable to the annual risk of death from all causes. Coronavirus infection though is a once per lifetime risk, whereas all-cause mortality occurs every year.

Just as a random comparable, the annual risk of getting cancer if you're 35 years old is around 0.1%, and the annual risk of having a heart attack is around 0.5% (higher for men, lower for women). All of those diseases would have similar average lifetime lost as covid, but the risk is continuous, not a once per lifetime occurance.

Sounds like they took the life expectancy of certain age groups without taking into account individual preexisting conditions whatsoever.

Not according to the article: "Then the authors accounted for other illnesses the victims had, in case they were unusually frail for their age."