There really is a different between being able to evaluate and implement designs and being able to build and connect functionality to the backend.
Specialising in a11y, i18n, CSS, etc. is a whole other ball game compared to hooking up interactive functions to APIs.
But I’m not a designer who knows how to code, I am a design engineer. My brain very much tends towards things like design systems, reusability, reducing repetition. Not to say I’m a crazy capitalist, who’s after productivity at all costs, not at all. I just want to reduce or remove the menial work that we have to do, we are amazing creative creatures, so we shouldn’t have to do that kind of work.
Some great ways of thinking about it
A front-of-the-front-end developer determines the look and feel of a `button`, while a back-of-the-front-end developer determines what happens when that `button` is clicked.
A front-of-the-front-end developer is a web developer who specializes in writing HTML, CSS, and presentational JavaScript code.
A back-of-the-front-end developer is a web developer who specializes in writing JavaScript code necessary to make a web application function properly.
The same separation applies to the backend:
A Front-of-the-backend person writes PHP/JS/etc. scripts which perform the required actions such as inserting records into a database, sending off an e-mail, etc.
A Back-of-the-backend person provisions and maintains the infrastructure.