The already have that in RDS. This is a da add of their own design. They were pragmatic enough to use the query language that most devs already know and trust, but how they pull off reads, writes, and sharing at that scale must have fundamentally different internals than a single MySQL instance.

It certainly seems to be a MySQL 5.6 fork (or at least backend ala InnoDB or similar), as the docs explicitly call out MySQL 5.6 compatibility. I feel like I wouldn't say "simply" though; that makes it seem like some guys at Amazon checked out the MySQL source, deleted some stuff and then released this.

Unlikely, because they already have a hosted MySQL 5.6 product in RDS. If anything, this is a MySQL 5.6 frontend + query processing engine with a new storage layer.

It would have been extremely difficult because MySQL community edition is GPL and Amazon does not allow GPL code to be modified by the internal teams as far as I know. They might have changed their policy though...