It may or may not be memcached -- it merely speaks the same protocol. I would not be totally amazed if something else is running underneath now or in future.
In the FAQ it states "Each Cache Node runs an instance of the Memcached software and has its own DNS name and port". So it looks like it is running memcached and not just talking it.
That is definitely plausible, but I'm more referring to the feature set, i guess i would clarify as the contract between them and the developer. That is basically memcached, from what i understood from their blog posts. As far as memcached itself goes, i hear it does what it does quite well, but that's ot.