The other way to think about it is that the bill may have saved your life. A shot that is hard enough to break the bill might be hard enough to break your skull and knock you out cold if it had hit you in the face. The bill gives some face protection. Not nearly as much as a face mask, but more than no bill.
So... maybe you just got lucky, lucky, lucky. You just nailed a rock full force with the face side of your skull and lived.
In a plastic helmet I think that the "give" in the plastic is what gives superior shock protection. I don't creek in a hard shell helmet because I think wearing rock hard kevlar ,or whatever it is, on your head just transfers shock very efficiently to your skull. When a plastic shell deforms and maybe even breaks, it is absorbing shock without directly transferring it.
I like a helmet with a facemask for riverrunning stuff that is difficult enough to flip me. It sounds like you didn't tuck tightly enough when you flipped and you got away with it. I didn't tuck tight enough once when I flipped and I broke my neck. Kayaking is dangerous like that.