As far as I know it does support CMYK and CMYK proofing, though I don't really work in the print space. That said, my novel's cover did come out as intended. 🙂

There is also the aspect of what "professional use" means. For those heavily invested in the Adobe ecosystem, I agree, Pixelmator is unlikely to represent a genuine alternative, but it turns out many people have only been using parts of Adobe's suite of apps for their work. I'd argue that still qualifies as professional use.

https://support.pixelmator.com/pixelmator-pro-user-guide/adjust-colors/about-color-management