The bull-shit thing is a great argument... very convincing and mature...
I find interesting and 100% historical adding a pinch of unexpectedness to the game system. It seems you don't but i will refrain from saying your thoughts are bull-shit, as i can cope with people having different opinions than me.
Mack was indeed the advance guard of the Austro-Russian army and the rest were coming in support just as you say. The French army was not as massive, specially as it was dispersed all around southern Germany surrounding Mack. And i'm sure he controlled enough land and cities to gather some supply to survive while waiting for the Russians.
But he didn't. He surrendered more than 30.000 men barely having fired a shot, awed and demoralized by Napoleon maneuver. Things like that and much weirder have happened in history since antiquity (blunders, treasons, trickery, cowering... a lot of things can happen and did happen)
Of course all this can be a bug in the siege resolution code but that will not change my opinion about the result.
If this happened on a regular basis it will be a problem, no doubt. But IMHO, having an unexpected result like this once or twice in a campaign makes things more interesting.
Cheers