Interesting they're using tree-sitter for syntax highlighting. Obviously they can't run LSPs in the browser so this must have been an alternative. I'd be curious to see this get into mainline desktop VS Code though (and I'm not alone: https://github.com/microsoft/vscode/issues/50140 ).
And there's no reason they can't use the LSP in the browser as long as the actual language servers can run in the browser - pretty easy with most languages these days.