Well true and not true. Screw down alone is not the solution. Greased O-rings and/or flat rubber gaskets are required as well. In addition, it is essential to have a certain design and manufacturing quality with very small tolerance and a quality control that deserves to be called so. None of the aforementioned makes a watch waterproof, it's the sum of all which gives you a timepiece which in theory could be waterproof.
When you have all that, screw down makes a difference, because not only does it prevent using the chrono underwater, it also applies limited pressure to the gasket and makes sure that they sit tight. The Panerai crown guard lever though with a different technique, follows the same principle. On top of that, a tight fit between the tube's and the crown's (or pusher's) thread will limit the amount of water that needs to be detained by the gasket.
My personal opinion: If you buy a gen which is sold as waterproof, get it wet. If it fails, you've got a warranty. If the watch is a supposedly waterproof rep or a gen who's warranty has expired, have it tested. It costs very little, takes no time and gives you peace of mind....