I think that the R-value thing is key. The OP describes the neoprene socks he used as "thin." Neoprene works in wetsuits (e.g. Dave's wakeboarding) because it traps a thin layer of water next to your skin which then is warmed by your body heat. That water doesn't exchange very much with colder water outside the suit.
But for hiking I don't think that a thin neoprene sock has any utility other than as a waterproof and possibly VBL layer. (Perhaps a nice thick one would provide decent insulation in itself, but who could stuff them in their shoes?) If he wasn't wearing gaiters or somesuch then his shoe was probably pretty much filled with snow and slush, and as that melts… well, the heat of transformation has to come from somewhere, and the thin neoprene wasn't enough to keep it from coming from his feet. The stream water, while cold, was probably both more transitory and didn't require heat of transformation. Plus, in the water the neoprene may have worked like a wetsuit.
… at a guess.
So, it seems that I agree with Dave- a simple waterproof/VBL layer and wool socks might have served him better. And, logically, it makes more sense to wear the wool inside the VBL, but I could be wrong.
But what the other Dave said is also a VERY important point that is often overlooked: Don't constrict your foot. (The military is VERY big on cold weather training.) If your shoe/sock combination is too tight it actually reduces bloodflow to your foot. Bloodflow from your core is what keeps your foot warm, and constricting it makes you more vulnerable to frostbite. If my feet get numb from cold my first step is always to loosen my laces.
I would propose that it is difficult to find footgear that fits correcly both with and without a neoprene sock. If you are going to wear neoprene socks you probably need a dedicated cold-weather shoe so that it isn't too tight. I did this- I bought a shoe much larger than normal so that it fit with a neoprene sock over a wool sock.