I'm curios about this too, does the Mac Mini offer "scaled" options, like MacBooks do with their built-in display, when connecting to an external 4k monitor? I think it does some supersampling, resulting in higher effective resolution.
In my opinion 2x scaling is the only option and that leaves you with an effective resolution of 1920x1080 points - sure, it‘s sharp, but far from true 5k and more importantly the same screen real estate we used to fit into 20-24 incch displays for years. It‘s the wrong resolution for 27“.