The job postings made it clear that they were for a brand new game, so yeah...
Also, releasing a new title makes a ton of sense, actually.
1. Diablo III is plagued by a "bad game" stigma ever since it launched - though subsequent patches and RoS expansion did a lot to improve the game, many people still associate it with being a failure. It's much easier to remove said stigma by creating a new title and giving the franchise a fresh start, rather than investing resources into a game that some people will always consider to be bad, no matter what.
2. They can't make sweeping, massive changes in an expansion. Sure, they may add content and new systems, but the core of the gameplay and mechanics would pretty much stay the same as changing it would not only piss off a large chunk of players, but probably also be more complicated than starting from scratch - which is what they can do with a new game.
3. The game is old. Sure, D3 still looks and plays really well, but in a few years it will be hard to lure anyone to the game unless it has better graphics, more modern engine, more robusts systems, etc. Putting all of that into D3 is - once again - not really worth it.
Also, comparing D3 to WoW makes absolutely no sense whatsoever. They are two almost completely different games with completely different financial structure.