> GP is pointing out that it is advantageous to have a single language on both front end and backend. And somehow you arrived at “developers cannot be expected to learn more than one language.”

Because he considers it a "barrier" to have to switch between languages to do backend-only changes. Which is a completely different use case than the one you're describing.

For an SPA where you're essentially doing frontend on both ends, yes, that's obviously easier when you're only doing it once.

But that's not the general case, and I'd argue that transitioning between different mental contexts for components you're not regularly touching is always going to be a hurdle to developers, regardless of the language used. Blaming it on the language is an useless red herring.