I can't source the comment with a quick googling so take the recollection at face value, but I recall one of either the Pyston team members or Guido himself in an interview saying that he hadn't been involved so far.

As of release 0.2 he was referring to it as being "built by some of my colleagues"[1]. Looking at the list of contributors for the Pyston repo on github[2], I don't see gvanrossum among them.

[1]: https://twitter.com/gvanrossum/status/510154006564335616

[2]: https://github.com/dropbox/pyston/graphs/contributors