LightedLamp, I don't think that person you are quoting understands what we mean by "side-chain." There would be no storing of hashes via OP_RETURN, and no referencing of external transactions. A side-chain is just another block chain, but one which allows transfer of value back and forth between itself and the bitcoin chain (and other chains too). So you can move coins onto the side-chain, do whatever fancy transactions you want (e.g. distributed markets), and then move back when you are done.