There generally is foam in a car door. If only because it is cheap structural support with the side effect of temperature and sound insulation. You can see this any time someone cuts a car door in half (which is shockingly common on youtube because junkyards sell shit for cheap).

That said, there is usually a layer of metal between that and the drainage space because… it is a drainage space.

What they are probably confused by is that there IS a motor in there (to move the window) and rotten milk could potentially adhere to gears and what nots. But that also is why you have the rubber gaskety bit on the sides of the windows. To minimize the amount of liquid that gets in there.

So if your car is fucked to the point that someone can pour milk down your window? Then just hose it off. And then fix the weather stripping.

I am sure there are cars that don’t have any barrier between insulation/electronics and the drainage space (my money is on said cars being sold by a manufacturer that starts with a T…). But the vast majority do because… of obvious reasons.