Yeah, I don't believe that for a minute.
We tip $5 per day in individual envelopes. We arrange our stuffed animals (Stitch, Mickey, whoever the kids bring) on one of the beds, and put the envelope in their arms. Most days when we return, Mousekeeping will re-arange the animals (once, we returned to find that Stitch had climbed the mirror in our bathroom!).
We have always received towel dolls as a "thank you," and we've actually had a Mousekeeper say "thank you" to us on the day we checked out.
So, I really doubt that the Mousekeepers don't receive the tips themselves. Perhaps they are required to share the tips with the other Mousekeepers (like some restaurants do for all of their servers). But, I can't believe that Disney would take the "gratuity for service" away from the Mousekeepers.