A compilation of Dreamers Cloth earlier work came out on fairly well known label Not Not Fun (Peaking Lights, Dolphins Into The Future, 2814, Weyes Blood...) just two days before the Societas X Tape came out. It included excerpts from The Coconut Pearl, but not the bit used in the SXT.
Personally I think it's great that this (and in my opinion, none) of the unknown stuff in the mix is by Boards of Canada. This way many more artists are discovered, and now there's a whole back catalogue of Dreamers Cloth (for example) to discover.
I'm sure this is exactly what BoC wanted with this mix.