His code is art. And so is his book. It is beautiful, and at times insightful and fascinating. It is not "engineering".
E.g. take Camping (a micro-web-framework that fit in 4K of code) - it is fascinating as a demonstration of some of the things you can do in little space. It is beautiful as art. It has provided inspiration for a lot of us that finds Rails unnecessarily bloated.
It is also totally unreadable for ordinary developers, weird and quirky to the point that if someone had checked something like Camping into one of our work repos, it'd be a "ha-ha, now seriously where is your actualy code?" moment.
The Ruby community appreciates code as art, and is probably more than others inclined to blur the boundary between it and what we will accept or even embrace in production systems. But _why's stuff is mostly firmly on the art side.
There are exceptions, e.g. Hpricot was a nice first try that demonstrates some of _why's skill in terms of creating pleasing interfaces, and he deserves credit for that even though HPricot itself has been largely superseded by Nokogiri.