That quote was made when 4.1 was first released on PTRs. From what Blizzard has said since, they decided not to include firelands and to release 4.1 earlier. Zarhym posted about this:
"A good amount of development progress had already been made on Firelands by the time we decided it would be better to release it in the following patch. Because of that, a lot of information about what is now considered 4.2 content was already in the game files for the initial 4.1 PTR build. This just reinforces what Bashiok said about 4.2 being scheduled for release on the PTR shortly after 4.1 goes live.
Development for both patches is currently well in progress.
"
In other words they had a lot of the development ready to go, but decided not to include it later. This quote by Basihok also seems to indicate it will be released in less time than a traditional content patch with a raid:
"Our plan is to package features like raids, dungeons, and daily quest hubs into smaller content updates and release them as soon as possible, rather than wait and release them as large but infrequent updates."