This is repeated so often (and on some reputable looking web sites as well) that people believe it and propagate it, but it's such an over-simplification that it's effectively useless. The Cat C/D decision is purely financial and based on the total cost of repair vs the value of the car/bike.
Cat C - The total repair costs exceeded the value of the vehicle. A £90 car with a cracked headlamp costing £100 would be Cat C.
Cat D - The repair costs exceeded the value of the vehicle less the salvage value. e.g a £500 car with a £350 repair, but the Insurer would get £200 salvage, so it would cost them £300 to total loss rather than £350 to repair.
With fairings, tanks, radiators, engine covers costing insane amounts of money, a bike would not need to be structurally damaged to gain a Cat C. OTOH a frame or swingarm would only need a minor dent or scrape to warrant replacement, and the cost of that would almost guarantee a Cat C write off, even though the damage is not actually structural.