All of the technologies it references (e.g. Cocoa, SpriteKit, the various specific API calls it mentions) are all strictly OS X. I would have thought the name "Cocoa" is recognizable enough on its own to indicate OS X.

> All of the technologies it references (e.g. Cocoa, SpriteKit, the various specific API calls it mentions) are all strictly OS X

I've never heard of Cocoa or SpriteKit, as I'm sure is the case for many others who don't use OS X - when seeing the name of something for the first time nobody thinks "oh that must be for OS X"...

> when seeing the name of something for the first time nobody thinks "oh that must be for OS X"

No, but when seeing a few technologies referenced for the first time, your immediate reaction shouldn't be "I bet that's cross-platform" either. And since the screenshots show OS X and the referenced browser (Safari) is an OS X browser, it should be pretty easy to figure out it's an OS X thing.

I had no idea what Cocoa was before this. I still...don't know what Cocoa is.

Cocoa is the name for the set of Objective-C frameworks that powers nearly every OS X and iOS app.

That's how I figured it out.