It's from the same team (which I am the manager of), but it's not using that same codebase. In Semantic, you would have to write Haskell code to add support for a new language, and we've found that the declarative DSLs that tree-sitter provides are a lower barrier to entry. (Semantic also uses tree-sitter for the parsing step, btw.) We do still have plans for Semantic, but our stack graph code does not live there.
[Edit] Also, the stack graph implementation is also open-source, just like Semantic, and we do our development on the core algorithms directly there. The Python extraction rules have not yet been moved over to the public tree-sitter-python language repo, but that's on the docket. Future language support would happen directly in each language's public open-source tree-sitter repo.