I am a Pixelmator Pro user and this move does not surprise me. The app has a very "first party, use MacOSX the way it is supposed to be used" feel to it and their website has always looked a lot like Apple's. I can't imagine them ever wanting to port the code to another OS.

I purchased Pixelmator Pro years ago. I think I bought it for half price in a sale but even at the current listed price of $50 it is a steal. I am in not way a pro image editor but it has done everything I needed it to.