Does the web bulb factor only apply when you are wearing a wetsuit, and wearing ONLY a wetsuit?

And another question resulted from a local conversation last night -- and this is about a drysuit:
If a drysuit works by your body heat warming the air inside the drysuit, will it still work if there is no air inside?
Dave White prefers drysuits to wetsuits, and he also uses a drysuit when he has to work on big boats in water sometimes up to his chest. As he goes deeper and deeper into the water, the air in the drysuit rises to the top. He then pulls out the neck seal enough to let the air out, so he ends up being sort of vacuum-packed, making the material plaster itself to his body, which makes it easier for him to work.

I asked whether he still stays warm even working in the water for an hour or more, and he said it keeps him plenty warm.

Of course, the water down here is 70 degrees, so it isn't a very good test. But I just wondered if less air in the suit would mean less warmth.

I'm asking in terms of being on a boat, not in the water. If you were in the water in a survival situation, I presume you would want as much air in there as possible, both for warmth and for extra flotation.

But I have always been very uncomfortable in drysuits, because I feel bulky with all that extra material. The "vacuum-packing" idea sounds interesting.