50% of
what? The reporting of that study is useless as it does not include any explanation of the method.
Is that 50% of your total body heat

Well, then you would be hypothermic or dead!

Is that 50% of your
loss at any in any given moment

.
It is the latter that I assume it means and that makes sense
only in specific contexts.
There is a course, no way that a naked body will lose heat from the head area at a massive rate above it's proportion of body surface area. But Naked is not how we normally roll in cold conditions.
So on a body that is well insulated by clothing and shoes, or a steamer wetsuit, it is perfectly plausible that most of the heat loss occurring will be from the uncovered, uninsulated part of the body, be that hands and feet or head.
Heat loss from hands and feet is lowered by the body's own reflexes. Blood flow to the peripherals is reduced to protect the core. Hands and feet are considered 'expendable' in the context of your survival, but this does not apply to your head, which is obviously a vital organ and supplied with much higher blood flow, and hence potential for heat loss.
So, it is entirely plausible that a surfers body (and feet) enclosed in an insulating layer of wetsuit, will have quite a low heat loss per unit of area, and the exposed head will have quite a high heat loss that could indeed amount to around 50% of the total loss. In that situation, insulating the head with neoprene or hat/helmet will have a significant affect on body heat retention. Covering the face will help too, but the face does have a heat retention reflex to some extent. That is why your face goes so pale when you get cold. I can attest though, that also covering your face (and neck) with goggles and face/neck muff does definitely significantly reduce body heat loss when skiing.