I don't think they'd use Swift for GUI building. Ladybird's GUI's are platform-dependent (Qt on Linux, AppKit on Mac, etc). From what I understand they want to use Swift as a replacement for C++.
I haven't done a lot of Swift, but I did play around with it recently, and it seems to work fine even outside of the Apple ecosystem. It has a LSP you can use so you don't need XCode, they even develop a first-class plugin for VSCode.
The idea of a future where Swift can be productive on non-Apple systems is definitely in place. The reality is that it will be several years before the tooling is mature enough to consider for time/cost sensitive endeavours. That is, if Apple maintains their interest in that direction by then.
I haven't done a lot of Swift, but I did play around with it recently, and it seems to work fine even outside of the Apple ecosystem. It has a LSP you can use so you don't need XCode, they even develop a first-class plugin for VSCode.