You could also read what Stross wrote slightly differently:
> SARS1 has 20% mortality among patients, MERS (aka SARS2) is up around 35-40% fatal, SARS-NCoV19, aka SARS3, is down around the 1-4% fatality level.
"Among patients" would make me assume that Stross is talking about case fatality rates, not infection fatality rates.
1-4% is a reasonable range for CFR, even if it's too high for IFR.
(Some quick Googling after the fact also shows that while MERS had a CFR in the 30-40% range, its IFR was estimated at more like 0.3-0.7% -- roughly the same range as COVID. Large numbers of undetected mild cases were cited as the reason for the large difference.)