There are enough proofs available on the internet to believe that WhatsApp UWP is a thing and it is currently in development. Besides WhatsApp, Microsoft reportedly is involved in the project to help out the developers in taking full advantage of UWP platform. This will replace the existing WhatsApp on Windows 10 mobile which is based on Silverlight.
We currently have WhatsApp on both Windows 10 mobile and Windows 10 desktop. The later is just a Win32 app made its way to the Windows store via Centennial. This is an important move in order for mobile PC to get a success, at least to some degree. The current WhatsApp desktop app is not a native experience. It will require an Android or iOS device to unlock the capabilities on Desktop.
For a device like Andromeda WhatsApp will play a critical role. If not there in the store, there is no potential consumers and even businesses. Microsoft knows this pretty well and is working on to solve the problem. Although it is pretty damn clear that if the project becomes successful we will have just one WhatsApp in the store which will run everywhere. This will also eradicate all the complexities and will be ready for download to any Windows 10 device.
As usual, Microsoft did not comment on the news. After all, nothing is final can be shelved at any point.