Within a given region, if there is more gray matter, and it is more clumped together, then the distance to nearest non-gray matter will be larger on average. This is what the authors call thickness.
I thought maybe you had some expertise regarding how they processed the data that could somehow make this (extremely strange thing) happen, but you have not demonstrated it. I guess I do not know for sure what they did without seeing the code, but if that is what they measured it is the most bizarre thing I have heard of when it comes to analyzing MRI data.
Regarding your comment about how incompetent I am, and how extremely strange this MRI analysis is:
Measuring cortical thickness by MRI is a standard technique in neuroanatomy since 15 years ago. And really, a quick search of the literature would have shown you this. But I suppose that's too much to ask.
I simply cannot imagine what you think is going on in order for them to be looking at "clumping" of gray matter as a proxy for density of the tissue via MRI and calling this "thickness".
>"Regarding your comment about how incompetent I am, and how extremely strange this MRI analysis is"
I thought perhaps you knew some technical detail about their analysis pipeline, but you still haven't mentioned anything technical... so I just don't have any idea what you are thinking.
>"Measuring cortical thickness by MRI is a standard technique in neuroanatomy since 15 years ago."
Sure, here is the first article I found regarding "cortical thickness":
"The shortest distance between the pial surface and the white/gray junction is the cortical thickness at each point." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5561315/
That sounds exactly like what I would expect, nothing to do with density or clumping.