[oh'mike]

Yes, thinset will bond to plywood--however, the thinset under a backer board is really just a filler that prevents hollow gaps that can flex when walked on---

the roofing nails are doing the actual 'holding'--

If you want a good bond (which is not needed) look for a modified thinset approved for setting tile over plywood---or mix liquid latex with an unmodified thinset--