Typing a large legacy codebase written in a highly dynamic language is not as easy. Instagram is pretty large.

I have seen a very large codebase in a dynamic language at [redacted] that has been mostly converted to use a sophisticated type system. Certain core things have not been converted so far, though, but just marked with a lot of "proceed with caution" and "TODO" red flags. Making them typesafe in any conceivable way would break compatibility for large subsystems.

Sometimes a rewrite is the only way out of such a situation, but very few can afford it.