I would assume the main idea behind the round was to set fire to petrol tanks protected by armour plate. So in the event of a clean hit the round would puncture the armour and the phosphorous would blast through the hole along with what remained of the steel projectile.
More often though I'm sure the round hit the aircraft skin, light structure, or some piece of equipment in the aircraft, began to tumble, and smashed into something more solid. At about 2000 feet per second it would be tough to stay intact at that point.
And really 'disintegration' in this case is simply the state of no longer being in one piece. Even just the light-alloy base cap has to pop off and Newton will do the rest.