@liaizon @evan @Gargron @chrismessina @eudaimon @youronlyone Yep, it was him.
It was first uploaded, AFAIK, in Wiki Commons by Eukobos (I think it's Eudaimon too), dedicated to the #PublicDomain through #CC0 #CCzero.
It was June 2018.
No one submitted other proposals so it became the accepted logo of the #Fediverse. Variations from other users based on the same logo also came out later.
See: https://commons.wikimedia.org/wiki/File:Fediverse_logo_proposal.svg