Depends what you mean by “fork”, really. I could clone it down, change something trivial, and run that on my phone for my own personal use. And that would qualify as a “fork” and be allowed by the license.
But I can’t disagree that a well-maintained, long-lived, publicly-available fork (like, MPV was forked from MPlayer2 or Libreoffice from OpenOffice) seems very unlikely.
And I’d doubt a third party accepting donations even if they operated on a non-profit basis could be done given the wording.
What theoretically could be done is that a FOSS drop-in replacement for GrayJay (even one compatible with GrayJay plugins, many of which I’ve heard are actually FOSS.) Though anyone who undertook that would have to be careful to make it happen in a proper well-documented clean-room fashion.