I thought Zed was using tree-sitter: https://zed.dev/blog/syntax-aware-editing? Shouldn't it address all of these issues? Does tree-sitter not understand Python (basically the most popular language out there) and Rust "beyond superficial syntax"? I thought its whole point was that it understands everything about a language's syntax because it builds a concrete syntax tree?

TreeSitter is an amazing tool but is (purposefully) quite limited compared to an IDE--it doesn't even cross file boundaries, so go to definition is a non-starter. Zed uses LSPs like Rust Analyzer to fill that role.