Yes, it is a waterproof membrane, but when you're riding, you know as well as I do that you are not completely sealed. If you are in a really heavy downpour, since your garb is not air tight, you're gonna get wet.
I use 'water-proof' ski gear and the same holds true...enough moisture will get you wet. You can wear Gore-Tex ski pants but if you sit on a wet bench, you'll get wet.
Waterproofing is now determined by a certain permeability factor...I do not know the formula, but you want at least 8,000mm waterproofing...the higher you go, the more waterproof you'll be...I have a ski jacket that is 10,000mm and ski pants that are 15,000mm...I still get wet if there are wet conditions...
On a bike with a serious downpour, you're gonna get wet no matter what you do!
Your boots might be waterproof while you're sludging around in water/mud, but if you're on a bike and water is coming in from the top, you'll get wet...you get my point...