If you take it at face value, the TrueTime API is implementable on top of any clock synchronization protocol. It's just coincidentally true that if you're close to a stratum 0-equivalent clock, your skew will be low. In terms of implementation, TrueTime daemons essentially also use Marzullo's algorithm to estimate timestamps, just like NTP.
Amazon TimeSync is probably a lot more than NTP as well. It must be part of XEN/hw as it is not yet available on the Nitro based instances. That it provide's an NTP endpoint is just the top of the cake...