will keep water out of the plywood during cure, and if anything spills later (assuming the crete is permeable.) My point was really about temperature on the other side of the plywood and any insulation layer(s.)
A masonry bond directly to plywood is not recommended. Especially one that changes significantly, thermally. You might consider several products made by this German company. They make excellent products for improving the bond between plywood sub-floors and masonry or ceramic finished floors. These products add some minimal thickness to the final floor. Not sure how they interface with pex, i.e. fastening pex down through the membrane.