I don't believe they actually did any reverse engineering for Windows Phone Link. iOS makes SMS/iMessages available over Bluetooth as part of its support for the Message Access Profile [1], intended for sending messages using a car infotainment system. This requires a physical iOS device to be located in proximity of the Windows device.

[1]: https://support.apple.com/en-us/102842

It works... "ok" but doesn't handle group messages. I find sometimes it just doesn't connect. They do post frequent updates though so there's clearly an active team managing the app.

I love being able to easily send URLs and other copy+paste items to my iMessage contacts from Windows!