I haven't seen the context of the original quote, but what you've quoted here is a valid statement and doesn't really have anything to do with what you're saying.

I'm in no way advocating client-side-only validation, but form validation on the client side to clean up mistakes can save a server round trip and a page reload, and so could save server side processing.

EDIT: just to clarify, I don't mean to turn this into a discussion with further nitpicking. I just thought your quote and comment were representative of some of the W3Fools content; picking a quote and interpreting it to mean what you want it to mean, and then criticizing it on that basis.