The author is an assistant professor of history.
These estimates are as reasonable as any other, and Youyang Gu had an excellent track record (he’s an engineer by training, not an epidemiologist). His work was so good that the CDC included his estimates in their model ensembles.
And yes: much of this is not complex and does not require extensive and specialized domain-specific knowledge to figure out. As documented here, many of those with expertise in the subject area have been wrong, by a lot.