He meant that after reading it, it was unclear that it was actually a legal license that would stand up in a court of law. It's just too confusingly written. See Nelson Minar's comment at the end of this: https://web.archive.org/web/20081015194558/http://radar.orei...
Every IP lawyer I've talked to since has looked at GPL and said something pretty similar. "It's just too confusingly worded to understand as a defensible license"