You know what else would communicate that? A function or macro.

    else if (tk == Mul) { applyOperator(MUL); }
    else if (tk == Div) { applyOperator(DIV); }
    else if (tk == Mod) { applyOperator(MOD); }

But then you're not conforming to their arbitrary idea of "minimalism = fewer functions".

I definitely have some admiration for their picking a goal and following through on it, and there are a few tricks in there that are downright brilliant, but let's not pretend this is about effective communication.