I don't think you actually need Perspectives to take advantage of the issue itself, since it comes with proper symbols for both walls and floors. But you need it to use the 3D projection command, and it also gives the advantage that you can draw additional isometric shapes manually.

Well, I guess the climate fits the bill for the Kalmar Union....

Ralf had a blog series about this:

He also has a live mapping session with the topic:

If you own the Tome, there is also a tutorial on this starting on page 154

Do you have Perspectives 3 installed? It is required for those features.

This is probably something that needs to be taken up with the original designer of the map. The FR Atlas map is just a faithful reproduction of an actual product map

The scale are from the original Myth Drannor maps from TSR.

The original Ruins of Myth Drannor Map specified 1 inch = 100 '. The non-ruin map that the one you posted is based on doesn't specify a scale as far as I can see, but it makes sense it is in the same scale as the ruins map, since it is the same place.

I looked at the original maps, and made some clips from the area around the building in the bottom left of your clip. Based on how the artist drew them, it certainly seems looks like they were drawing them like individual, to-scale buildings. But I do agree with you, the sizes is a bit large. So I'll probably chalk this down to a bit of a poor quality control with the original design. Lots of maps from those days played a bit loose with scale.