I had the fears when I saw "Perform precise textual or regular expression searches" next to "Track down every definition and reference of a specific symbol" but it seems they're kind of using tree-sitter, but only if you happen to name your files in the magic way
https://github.com/cased/kit/blob/v0.1.1/src/kit/tree_sitter... against these versions of tree-sitter
https://github.com/Goldziher/tree-sitter-language-pack/blob/...