Maybe we're using different definitions of "native". In the most common case, both of these editors use the cross-platform "VT-100 UI Toolkit" (e.g. ansi escape codes in your terminal emulator), regardless of platform. Some exceptions do exist. But native as it's being used in relation to this project refers specifically to whatever frontend framework is 'the standard' for a given platform, where one exists.

They aren't webapps but they're each a different ecosystem and workflow entirely separate from the rest of the OS. You can add integration to various parts of the OS via extra scripts/plugins but that you need to do so proves the point. With Xi it seems the goal is to have a powerful shared core but the UI frontends are tailored for the OS/DE they're meant to be run in rather than having one generic cross-platform frontend.