on relates to the surface, even vertically

I'd say it's similar to: On the side, On the waterfront, A house ON the lake (doesn't literally mean it'a a boat), etc. A shop on Main Street (it's actually next to the the street) In kind of means ALONG, NEXT TO, NEAR etc.

For a free standing wardrobe "up against the wall" is better and more natural grammar. The text does not appear to mention what type of wardrobe, so it could have or might have been hung on the wall.

It is probably a built-in-cupboard. Which is why it is 'on' the wall. However, the grammar of the passage isn't the greatest!