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.
LowePro Dryzone 200 bag - CLICK ME!!
A User Review of the DryZone 200 bag - CLICK ME TOO!!
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