That is one way.

What I meant was transpiling Luau (in memory or cached to disk) -> TypeScript -> typecheck with tsc -> take error outputs and line numbers -> transform back to Luau code via sourcemaps etc. This is potentially way easier than making your own checker for another structurally typed language.

User only sees Luau script in their editor, but it gets checked by TSC in the background.

Roblox might is such a big maker that they can re-invent the whole structural typing themselves, so they don't need to do that.