If you're sitting on the opposite side of a table looking at a map that a person on the other side laid out in front of them, you don't just see the map from the other side? You instead see details about volume, shape, distance between points, geometric similarity, colour, and so on? I sincerely just see the same map even if I'm across the table, except flipped. I'm not sure how it would impact my drawing a map, though that isn't really what the article talks about.

Can you read upside-down or does it become a jumble of lines? I can read upside-down with no special effort so maybe this is canceling something out.

I can read upside down, but it's an acquired skill. I can't read at a 90 degree angle without difficulty, although I could probably learn to.

It's not that I don't see the map from the other side, it's that when it's the right way up I see all the extra information I have about it. For example, I bet an eye tracker would show me focusing on Western Europe, Central Asia, Australia, and the US. When the map is flipped, I see it closer to how it really is because I can ignore those preconceived ideas more easily. I don't see e.g. the Iberian peninsula as represented by a land mass, I see the actual land mass, and can concentrate on its size and distance more easily.

This is really interesting!

I agree. In addition I can relate to elements (such as imagining the road from Paris to Munich), it just takes more processing.

I too have no problems reading upside-down, but for some reason I do find it hard to read sidewards.