So first, off, the more recent version of this page is here.
Regarding you original question, it does seem like this was perhaps a mistake. @aloctavodia might weigh in (though he wrote the original notebook a long time ago). Alternatively, you (or someone) could check the McElreath book itself (where this example was taken from) to see whether he changes the priors in this way. If not, we would welcome an update. This is exactly the kind of thing we hope to get done at the upcoming PyMC Docathon (detail here).