This is certainly an unexpected move, but not necessarily an outlandish one. Why would Apple bother making its own controller for Arcade when a solid option already exists? Of course, one could also argue that Apple may have been better served by partnering up with Sony and snagging the DualShock controller instead of working with such a direct competitor.
Anyway, while we don't know what sort of discussions (if any) took place between Microsoft and Apple to make this development a reality, it's good news for consumers. Having another place to buy an Xbox controller can only be a positive thing, though the device is reportedly only available on Apple's website for now (we don't know when it will arrive in stores).
We should point out that Apple's website is currently charging a bit more -- $60 -- for the controller than other storefronts. Amazon, for example, has multiple official Xbox wireless controllers for sale at the $50 price point; and that includes free one or two-day shipping in many cases.
The gamepad being sold on Apple's website comes in black, and uses Bluetooth for its connectivity (no adapters necessary). Two batteries are provided in the box, but the store page makes no mention of any other charging accessories.
If you already have a Bluetooth Xbox controller and want to hook the device up with one of Apple's products, watch the Apple-made video above to learn how the pairing process works.