I don't know the details, but I know they emphasized that the three(!) platforms share a lot of common frameworks/foundation. Basically, when the iPhone was introduced, Jobs said "iPhone runs OS X." That hasn't changed, and I believe the watch does to a certain extent.

So from a programmatic standpoint, they are three platforms with one foundation. And Apple isn't going to proclaim "one OS X" any time soon since they want to emphasize (and make sure developers are aware of) each platforms' differences: iOS for touch interactions and limited multitasking, watchOS for limited interactions (glances), OS X for power users and doing things not possible/practical on iOS. But all three are programmed with Objective-C/Swift and use some Cocoa-like framework.