Putting things in the public domain voluntarily is indeed more difficult than it should be. The best tool we got is CC0, but it still raises (probably unwarranted) concerns for software.

This is unclear on the technicalities (is it even possible to just claim it like this? is CC0 required? etc.). Still this is a bold move, hats off to this renowned author.

Indeed, without deciding to put everything in the public domain, the face of the web would have been very different.