https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-fallow-ground.htm#:~:text=Fallow%20ground%2C%20or%20fallow%20soil,five%20years%2C%20depending%20on%20crop.

i.e. appears to be unused farmland?

Its not location based. It just means the total amount of fallow lans in the contiguous US is about equal to Michigan.

100 people don't own Florida and more.

I assume it's stuck in a court process, or in the process of transitioning to another status, or farmland without a crop, or maybe abandoned but the city can't pay to clear it or demo it yet, etc.

Idk didn't bother going around the pay wall just thought it looked cool. I'd guess the area of land that is idle/fallow is the size of michigan