They're not saying your manual copy/pasting from a browser window onto your clipboard shouldn't be possible; they're saying the browser itself shouldn't allow this functionality to be scriptable internally.

The former is a system event, not a browser one.

This is similar to how on google docs if you try to copy via the edit menu, it pops up to say use Ctrl-C instead.

Honestly, I too am surprised a JavaScript plugin was able to access and alter the system clipboard in the first place too.