I still don't see where it fits into the lineup. If there's still a PixelMator Pro in 6 months, I'll be surprised, but it won't be the first time I'm completely wrong about something.
Being able to put Shortcuts into Control Center in iOS 18 is a handy option, if anyone missed that you can do that now.
If I was to place a bet: they are looking for the next graphics killer app for ipad and macos. They purchased Pixelmator because Pixelmator owns the intellectual property for all their code and Apple found it more convenient to buy complete IP rather than reinventing the wheel.
Bringing a more premium experience to Photos would be a great complement to how you can already shell out to Pixelmator while editing photos.
I'd put money on this acquisition being used to improve the image editing experience in photos.app on iOS/macOS, just like Dark Sky was acquired and then used to improve weather.app, rather than any return of Aperture.