is this product also considered a moisture barrier if i use it in a bathroom before laying self adhesive tile over it?

I would suggest to look on the label and see if the product is water proof. If it is, then it should be good as a moisture barrier.

DAP® Flexible Floor Patch and Leveler is "ready-to-use", latex-based patching product which is designed to fill cracks, fastener or knot holes and level low areas in plywood subfloors or concrete surfaces. The floor patch and leveler will remain flexible allowing the plywood subfloor to flex without cracking the filled-in areas. Ideal for filling and leveling surfaces prior to installation of carpeting, vinyl floor tile or sheeting, as well as ceramic or mosaic floor tile. It is not designed to function as a moisture vapor barrier or waterproofing membrane.