Earlier quoted context omitted.
Nonsense. You can migrate a back end developer to the front end in far less time than it takes to train a new front end developer from scratch. The domain knowledge of front end specific tasks must be learned, sure, but the core fundamentals of how to program remain. You don’t suddenly forget how to actually program once you pick up JS, all jokes notwithstanding.
True for the programming part of front end development, not so much for the making things look good part.
[0] https://en.wikipedia.org/wiki/Narcissism_of_small_difference...