I can understand (although I'd still think it's a bad idea) going with a web app wrapper if you're starting out that way, but what makes this decision truly baffling is that they already had a UWP app.

Meta makes more money than god and there's over a billion WhatsApp users. It's not like this thing is Blender or a AAA game, it's a chat frontend. Maintaining it has to be a rounding error in the budget.