If they're building a native macOS UI, how is it ever possible it'll be worthwhile to port that into native Windows UI or "native" Linux UI? If you wanna go cross-platform, you usually consider that from the start, not once you're 50% done, as you're gonna have to redo a lot of work you could have gotten "for free".

The “engine” itself is likely cross-platform, with vendor-specific UIs.

It’s likely quicker to develop for a single OS and then build the additional UIs with a more feature-complete app.

Arc browser is also doing this, and they're planning on debuting the Windows UI in Fall.

Isn’t the whole selling point behind their thing that they don’t use the native macOS UIs but instead send everything to the GPU?

Rendering a platform native GUI and what you render it on are two different things. In the case of Zed, its a combination of - metal is quite easy to work with - apple has clear design guidelines and nice toolkits

The Xi editor team quickly found out how much of a mess "native" gui's were on windows. Zed will hopefully add support for vulkan and linux/windows later, but it makes sense to start off on macOS

Metal is not a platform native GUI, it is Apple's graphics API. The native UI on the platform is Cocoa (which calls Metal for some things, but isn't being used here).