Pls forgive my ignorance, but how is Typescript (a superset of Javascript) used to type-check Python?

You can write a parser and type checker for pretty much any language in pretty much any language. It's just text files as input and text as output.

They're saying pyright is a Python type checker, but it's written in Typescript, not Rust.

There's nothing magical to type-checking Python. You can write this in any programming language. TypeScript is actually a pretty nice language for writing static analysis tools.

just like the Python compiler/interpreter is written in C.