They’ve always had a pretty good relationship with Apple and the apps have always been very “Mac-like”,
I think that's the problem though. These days, most of Apple's products (anything outside of the Finder) is a UI disaster that just about completely abandons Apple's own HIG for design and using Apple-native and visually appropriate controls and ways to work.
So while Pixelmator is extremely Mac-like right now, and btw Pixelmator/Pro should be used as a very high bar and inspiration for other developers looking to make Mac apps. They use controls, layouts, and so on very effectively and it definitely feels like a "Mac App". At least for now.
Now that Apple has it, I have a feeling that it's going to get much less "Mac-like" in future releases, because Apple can't even manage to get much right in their own apps, they're all over the place and seemingly not really following any guidebook or design rules across their apps, or even in a specific app.
Man am I starting to sound like a troll in here lately. Things with Apple are just going downhill, have been for years, but seems to be ramping up as time goes on - in the wrong direction. So I am not such an Apple fan right now.
I still use Apple devices (and thus software and 3rd party software) and wish there was a better alternative, but sadly there isn't. MS/Linux are still far from being a good alternative for long-time Mac users.
So I definitely still don't think this will be a good thing whatsoever.