Please note that the mac app has been removed from sale temporarily because it is not compatible with GoodNotes 5. An update will be available soon.
Yes, the Mac client requires another purchase. It is not possible to bundle paid iOS and Mac app on the App Store. We've developed the Mac app from scratch and it will continuously be updated and supported in the future. Once you bought the Mac app, you can download it for free on all other devices that run MacOS. The same applies to the iOS version.