"At the time that I wrote this book I knew about 20 programming languages and could learn new ones in about a day to a week depending on how weird they were."
He's saying the entire range is a day to a week. The ones that are isomorphic would take a day or two, I agree.
But he's saying the ones that are not isomorphic also take just up to a week, and according to other discussions with him, it's not because he's already an expert in those other types of languages.
javascript, qbasic, java, c++, c, c#, ruby, python, perl, they are all very similar, some with their knobs exposed, some without, some with bad environments, some with better, some with closures, some without.
Once you grok the inner similarity of software, it's just not very interesting to learn new languages.