I thought Wagner's algorithm sorted. I'll have to reread the paper more carefully.
That puzzled me too. I don't see a need for sorting, just for binning to find collisions on the next chunk of n/(k+1) bits
In this case it turns out there is little difference between fully sorting and sorting into bins,
since the expected bin size is only 2 :-)
So equihash really seems to be all about sorting...