I love what Peter Grey has to say and I'm happy to see him published by a mainstream publication like Psychology Today. I saw him speak last year at an unschooling conference, and he'll be back again this year, as well. Anyone interested in unschooling and raising children would likely appreciate his work.
I would say that what is called "formal play" is not really play at all. Isn't it part of the definition of "play" that it be informal? By formal, I mean "managed or directed from the outside" and informal, I mean "self-directed," which (as you say) is another way of saying "statism" versus "anarchy."
There is self-knowledge after children.