That’s because these are 4K monitors. The 27" iMac comes with a 5K screen.
There are very few 5K monitors and they aren’t cheap. If you go back and check @jcenters’s article, you’ll find a link to the LG 5K display on the Apple Store. It’s $1300. Amazon has them refurbished for $1000.
This 5K screen would work just fine with a Mac mini.