Yes and no. It's important because of the way the fibres will tighten when wet. Ventile will tighten into a hard stiff cloth. It becomes very water resistant when it does so. The twill weave is a very strong fabric but it has a surface with only one third of the undershots that a basket weave (oxford) will have.
Grenfell cloth was very windproof but also very breathable. It was definitely shower proof and when very tightly woven pretty rainproof, but in persistant rain it would let in where double layer ventile would not.
At it's simplest, Grenfell was woven, over under over under over under, while twill ventile was woven over two under one over two under one.
Does this help ?
I think John Fenna's probably the best person to ask about this kind of thing, he has probably reviewed the stuff, and might even have designed clothing using it's properties to their best advantage.
cheers,
Toddy