It’s also what critical thinkers do when evaluating “what percentage full of shit do I think this author is?”

If a glaring innumeracy or terrible estimate is in the article, why did the author include that? What was their angle? Does that make me trust the rest of the article more or less?

Because it indicates dishonesty and/or innumeracy which calls the accuracy of the rest of the piece into question. "Checking if the author can actually count" is basic media literary stuff, not some sinister agenda.