Their description sounds like literal thickness. They took the distance to the border (white matter) of all the points in a region. This is like the radius of a circle, and then multiplied by 2, to get a value like the diameter of a circle.

It does sound like quite a convoluted process but I don't see where you are getting anything like "density" from that description.

Also, reading the methods closer makes me think these are cherry picked results:

>"Once segmentation is complete, the original images are interpolated by a factor of 7, resulting in a final voxel size of 0.39 × 0.39 × 0.43 mm. Next, up to 18 connected layers of gray matter are grown out from the boundary of white matter, using a region-expansion algorithm to cover all pixels defined as gray matter."

Why 7? Why 18? These magic numbers shouldn't be there without some kind of sensitivity analysis.

Finally, you can see the dimensions of the voxels is something determined by their methods, not a property of the brains.