From that 5-pass process the marked-up text is handed to a volunteer "post-processor" who assembles the final HTML or e-book file; then the completed book gets one more "smooth reading" pass before it is posted to PG.
This it the process that produces the books input to Standard Ebooks. That they can still find scanner errors ("tne" for "the", a typical "scanno") demonstrates how difficult it is to see those. But their presence isn't from carelessness or disregard for the value of the books.
In the 20-teens I put in hundreds of volunteer hours at PGDP in all the above roles, and it was very satisfying work. I'd recommend it to anyone wanting an online hobby that feels constructive. Volunteering time to Standard Ebooks would probably feel good as well.