Can you expand on what you meant by that?

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.

What am I missing?

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...