It's simple enough: before the rise of front-end frameworks in the 2010s, web developers were indeed more of a single group ("full stack"). But even before the rise of Angular in 2011/12ish, there was already starting to be a specialisation in front-end vs back-end; some devs, I remember, worked almost wholly on the JS UIs of more front-end-heavy websites (they may have been more of web designers previously). Others concerned themselves more with the backend, databases, services, etc. As frontend technologies grew more complex (and mobile became of primary importance), the division grew.

Before Angular and co., "full stack" encompassed using HTML, CSS, and JS. What it can be fairly said to mean nowadays is debatable, but 10+ years ago, doing Rails with a bit of JS would definitely have counted.