> It's not the same language, so why should he expect the semantics to be the same?

It's a bit like discovering that inline C in your favourite non-C language isn't actually semantically correct C. If you're a mathematician, then you know, intimately and fluently, how math is spoken and written, so seeing something that looks exactly like "inline math", but isn't, is jarring.

(I don't get angry over it, but it is exactly counter to the extremely delicate and precise way mathematicians train ourselves to think about '='.)