Not the GP, so not sure what he was specifically talking about, but wool is known for retaining its insulative value even when wet.
Yes, you're wet, but you're still warm.
Many outdoorsmen prefer this to the uncertainty of being able to keep their gear dry.
Yes, that's what I'm talking about. By thick rainy weather I love nothing more than a coarse knitted woolen sweater and a heavy tweed jacket. It's warm, breathable and I'll never stay so long under the rain that it's soaked.
As far as strictly waterproofing goes, you do need coating or some hybrid material with PU weaved in. IMO it's more useful as a wind barrier than anything else.
What sort of wool is good for keeping off the rain?
Edit: Wearing (only) a damp wool jumper is fine if you’re up a mountain and it’s not too windy or freezing cold, but I’d still prefer to be dry!