From everything I've read, yes, that is correct. Only Sony Pictures has been targeted, only their emails leaked, only those employees threatened, etc.

Also, you have to understand that Sony is a 140,000 employee organization, with many different operating units that are functionally probably pretty separate. The Sony Pictures network and email system was likely entirely separate from other Sony divisions, especially since one was based in Los Angeles, and others are probably based in Tokyo.

If it helps, Sony Pictures Entertainment is just the new name for Columbia Pictures after Sony bought it.