THEN VS THAN

I had read on the Internet that English speakers confuse then with than. I found it difficult to believe mainly because the pronunciation of both words is different. Last night, however, I found palpable and visible proof of this in Bill Hayes's The Anatomist (2007, Kindle edition). I had read his Insomniac City (2018) mainly because it deals with his relationship with Oliver Sacks of whom I am a literary fan, and was, at that time, reading his On the Move. Lo and behold, I came upon this passage: "... I found something even more illuminating, though it is written so quietly it would be easy to miss: Offered own assistance. Carter than adds matter-of-factly, Gray will let do some." It sticks out like a sore thumb to the point that my spell checker has underlined than in red as I typed it. Mr. Hayes meant then, not than. So it is true that educated, well-read speakers of English make this silly mistake. What is worse, his literary agent, his editor, and his publisher have not noticed this blatant error. Horrors and procrastination, I said to myself! Live and learn.