Taken in the spirit of the question, as far as I'm aware they are. I only use the ballistic nylon ones and those are definitely waterproof, and from what I remember there's not much difference in terms of features except that the ballistic nylon is supposed to be more durable. At any rate, a quick web search should answer your question.
Its a matter of semantics and using the proper terms.
Waterproof - Impervious to water
Water-repellant/Water-resistant - Prevents water absorption
So, to be correct, I would say most (99%) of camera-bags are only in the water-repellant/water-resistant category and are able to (in varying degrees) withstand rain.
From personal experience:-
Lowepro Mini Trekker - caught in rain shower, the outer nylon is really wet with a little bit of dampness inside but did not seep thru the sponge.
Lowepro Nature Trekker with AW cover -again caught in rain shower, but using the AW cover, the outer nylon was slightly damp but no dampness inside the bag. If didn't use the AW covering, probably would have experienced the same thing as the Mini Trekker.
For a truly waterproof bag, only the Pelican dry cases and the Lowepro Dryzone 200 fits the bill - Impervious to water, even if dropped into water.
So going back to the original question - canvas Domke bags are probably very low in the water-repellant category but can be enhanced by using one of those Scotchgard spray-ons (forgot whats the name already). I wouldn't categorise the Domke canvas bags as water-proof
I've just done a search for Domke information on the web and the best I could do was B&H (okay, so I didn't try too hard). B&H clearly states that the Domke bags are made from breathable, waterproof (resistant, whatever) canvas, and occasionally ballistic nylon. The waterproofing is not conditional on it being ballistic nylon or not.
From my experiences with my Domkes, (as mentioned all are ballistic nylon but this shouldn't make a difference from the information on the B&H website) they are water resistant perfectly well. I shoot in all kinds of inclement weather, and my Domkes get standing water left on the flaps when I have to cover games in the rain, but the inside remains absolutely bone dry. Certainly better than my Lowepro stuff. The problem with Domkes is that they don't appear secure because the bags don't zip up tightly, but  the flaps overlap so unless you do stupid things, it's impervious to falling rainwater, and  that's nothing a Scotchguard spray on can help with that either.