The reference is to the metallic coating on the glass that makes it colorful. If you fuse irid side to irid side, the glass will likely not fuse properly. You can certainly fuse non-irid glass to irid glass, or the non-irid side of irid to an irid piece.
When fusing irid to irid, the trick is to remove some of the coating so that the layers fuse, as shown in the tutorial.
Jester’s Baubles Fused Glass Designs