It's common to call the Symfony framework a Full Stack Framework. This goes back to the days of it providing everything on the server side, from templating with Twig to data persistence by integrating the Doctrine ORM. But in the job market the title Full Stack Developer today means someone who is fluent in working in both front end and back end. With Symfony UX the framework is updated to be closer to that definition.
Symfony UX provides developers comfortable with backend domain with PHP and Symfony to interactivity to the front end with preferred tooling and documented conventions. With server generated core markup, you could call it back end driven front end development. This will certainly not be something for everyone, but it need not be. You are free to use Symfony as your backend for a pure API or use Webpack Encore to build a full stack app in a monorepo with React.js client app for example.