The boxes are made by Arris (previously by Motorola) so it's just the Mediaroom and MediaFirst software including the back-end plumbing. I read that there are over 40 million STBs running Mediaroom worldwide and they just announced a small-footprint version that will run on various under-powered legacy cable boxes with a lot of the processing happening in the cloud.
Apple restricts what an app developer can do, so it may not be possible to provide the tight integration with the PVR that a developer can accomplish when he controls the software down to the bare metal.
MediaFirst has been around for quite awhile before that agreement was announced. It's not clear what the back-end software powering the Fibe TV app was previously.