I believe macOS only renders at 5K if you set a 2560x1440 UI resolution
If I set the "looks like resolution" to be 3360 x 1890
then in System Profiler it reports the GPU is rendering at 6720 x 3780
So it is always doubling the visual resolution, then downscale it back to your panel's native resolution.
Some folks in this thread are arguing as long as you don't scale by integer then you lose so much sharpness it is unacceptable, therefore deeming most of the 27" 4k displays out there not real alternatives. (which I don't agree but to which their own)
The typically accepted visual resolution of a 27" display at desk distance is probably 2560x1440, so that the 5K with 200% scaling is a kind of de facto standard, at least Apple keeps putting out 27" 5K iMacs they must be thinking along that line.
The problem is 4kUHD being the 16:9 video content standard by proxy, and game consoles as well, therefore the market is driven to make 4kUHD panels almost exclusively.
At this point in time we are seeing so many "good" 27" 4k alternatives including ones mentioned in this thread, some have clearly better specs than the Studio Display in one way or the other but never at 5k resolution. I guess it depends on which you value more, sharpness or other attributes, especially concerning color critical professional workflows.