The official definition of "open source" requires allowing forks and derivatives that are closed source.
I have no idea what "official definition" you are referring to, but that is certainly not what the Open Source Initiative defines as open source.
Nothing on that page specifies that an open source license must allow closed source derivatives. The GNU GPL is even explicitly listed as a fully OSI-approved open source license.
You are correct that the FSF considers "free software" to be something more than "open source", in that "free software" explicitly forbids any kind of closed-source derivative. But that doesn't imply that "open source" specifically requires that closed-source derivatives be permitted.