Re: "you can't overfeed at the breast"
That statement can be confusing but you're right it has to do with nipple flow. A baby has a high suck need and will keep doing the motion even after they are full, usually for comfort. With the breast they can alter their suck to get little to no milk. With a bottle the milk keeps coming and they end up eating more than they need.
You ask about oversupply and that's something slightly different. The type of milk a baby gets is not as rich. Think about what it feels like to fill up on water as opposed to food. It won't last as long. Plus it can cause gas and discomfort for the baby.
If you obey all the rules, you miss all the fun. - Katharine Hepburn