Cassandra in multiple data center - EC2MultiRegionSnitch
In regard to a standard implementation of Cassandra the answer is Yes. So everything in the cluster regardless of the data center would have access.

From the Apache Cassandra site:


  Cassandra's support for replicating across multiple datacenters is
  best-in-class, providing lower latency for your users and the peace of
  mind of knowing that you can survive regional outages.


The site you're referencing is talking about a service they perform called partitioning where they're actually distributing data in a different way based on their partitioning scheme, while still using Cassandra as the database. They do replication based on NetworkTopologyStrategy which depends on the network and the number of datacenters.

In regard to Amazon EC2
They're mixing lingo a little bit which is where the confusion is coming in.


  ... on Amazon EC2 where all nodes in the cluster are within a single
  region. The region is treated as the data center and the availability
  zones are treated as racks within the data center.


So on Amazon's network a region will contain multiple data centers, but they're distributing info like it were local to a data center (probably because their network has plenty of resources and nodes). This document here goes more into the availability zones concept for their network. Basically their Network Topology is a little larger than most networks.

The data should remain in what they're considering a data center, but whether this is the same physical data center or not would depend on the number of racks, the types of machines they're running and the backbones (OC-3,OC-12,OC-48, etc lines) going between them. If you're in a region where they can't have replication across multiple data centers as quickly they'll likely have more redundancy in the physical data centers themselves.