It only is a valid statement if you add the notes about always doing the validation server side too, and only using the JS one for improve UX. But w3schools is

It only is a valid statement if you add the notes about always doing the validation server side too, and only using the JS one for improve UX. But w3schools is read by beginners, they don't have any clue about web security whatsoever, if you don't tell them everything they will just go with the few things that you've told them.