That's because this isn't your old EU Interdictor at all...
The EU Interdictor was a medium-sized support cruiser.
The new Rebels Interdictor, of which this is, is more akin to a heavily modified Star Destroyer. It was a Prototype in the Show, a Testbed going through final operational testing by ambushing Rebel cells and swallowing up CR-90s like some Tantive-Devestator Nightmare yet to come...
There's nothing to state the Immobilizer 418 Interdictor Cruiser won't come at a later time. It too is canon as of the Tarkin novel. But for now, we get the 'fresher', more 'visually new and familiar to newcomers' Interdictor.