Personally, I've never had any trouble with setting tile directly to a wood surface, but I can see where if it was an OSB type of board (chip board rather than ply) it might be recommended, as this product will start flaking if it get's very wet. It could also be in concerns of water from spills or cleaning getting to the wood through the grout. Someone else here might have other thoughts on this.
The mortar mix you have is the right thing. NOTE: Although this mortar has a pretty long pot life, don't try to mix too much at one time unless you've got a heavy-duty 1/2" drill and paddle to mix it. It's a real chore to mix by hand, but can be done, just don't try to mix huge amounts all at once; you'll never get all the dry clumps out of it.