You should read the source you are linking to instead of being needlessly snarky.

The function you call emits IR, and it's doc block says:

    // EmitIR compiles the Lua script similarly to Compile but instead
    // uses the long listing options to capture the compiled IR dump.
    // EmitIR returns the Lua bytecode encoded as a string or any error.

luac is the Lua compiler, not a reflection of what language the engine is written in. I'm not saying it doesn't call a compiler written in C, but executing a binary named `luac` doesn't mean that it does https://www.lua.org/manual/5.3/luac.html