Indeed. These aren't very complicated subjects, it's easy enough to learn and doesn't require full-time work to become an expert with (and the same goes for backend web development), it merely requires constant practice and dealing with production.

The constant practice and dealing with production your java developer would not have unless that became their job focus.

So are we agreeing that being extremely broad is an anathema to quality because you cant possibly constantly practice will all things?

No. You can be a senior java developer with great expertise with only a part-time job. There's nothing that prevents you from being at a senior level in multiple technologies or fields of study. More then that though, doing both complements each other, so that a senior level full-stack developer often has better insight about server-side issues then someone who only ever knows how to provide back-end solutions to product level problems.

yes!! That is what full stack is.

You can, if needed, produce a working app working from the database all the way to front end. This includes, design, ops and development. All of it.