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.
The classic example is of course validation, but plenty of other places it can come up.
But as you can see, there's n+1 standards for it, and in practice none of them has a Javascript implementation good enough for productive use. So it's more a "what if?" scenario, had Angular/React/Vue not taken off.
Also, what if you just have frontend developers who straight up don't know Java or vice versa? I don't see how having a shared language could ever be a bad thing.
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.”
There ARE big advantages to having the exact same language (as opposed to similar languages) on both client and server, especially in an SPA where you want to render something initially on the server, but then update it using the same logic later on the client.