I don't tell people I'm a full-stack engineer because I'm equally good with every random piece of technology at use in a product, but because it signals willingness to apply the imperfect human skills that I do have to different parts of a system.
The front end of a single-page React application is a lot more complicated than just HTML templates and CSS, and you can get all sorts of productivity and developer happiness gains from being able to jump between front end and backend - consider implementing the network layer in Redux, breaking out validation logic into a shared library, or fixing a bug that can only be understood by looking at both a front-end request and the endpoint that handles it.
The front end of a single-page React application is a lot more complicated than just HTML templates and CSS, and you can get all sorts of productivity and developer happiness gains from being able to jump between front end and backend - consider implementing the network layer in Redux, breaking out validation logic into a shared library, or fixing a bug that can only be understood by looking at both a front-end request and the endpoint that handles it.