Their decision was because of all the AI slop. It was becoming difficult to understand developer intentions with big PRs that could obfuscate malicious intent. And developers making PRs just for the sake of making a contribution for their resume than actually helping the project. Similar to the developer of curl having to take a few steps back because of the overwhelming AI slop out there, though in the case of curl it was bogus bug reports. But they are still developing in the open, just only allowing contributions from team members. And this doesn’t prevent porting efforts, but might make it impossible for Ladybird to have upstream Haiku support.
As for Rust, they wanted a memory safe language and found that most Swift components were macOS/iOS-specific and not really cross-platform (though Swift itself is).