The point was that a textbook (where the 40hr/page estimate comes from) is cumulative/linear -- what you need for page n was defined / established on the preceding pages. But in a proof such as this you can call on any other published result (and those can do the same) so the dependency graph is (potentially) much bushier. Thus later pages of the proof should take far more than 40 hours to manually formalize.

The scale factor comes from this number in the article, seemingly an intuited estimate:

> Say a research article takes 20 times more effort to formalize than page in an undergraduate textbook.

That would suggest formalizing a 10-page research article might take 200 weeks (assuming 40h/wk) of effort, or about four years. Not a mathematician, I have no idea if that's in the ballpark.