I though Photoshop UI scaling was a Windows preference it was introduced in CC 2014 as an experiment feature. I see it is available in my CC 2018 on windows. It is still full of issues and Adobe is working on a new implementation there is a beta test version you can sign up to test. I do not know if it includes Mac support. I never installed the beta for Adobe was not working on the scripting area that I was interested in when the beta was announced.
This is an old topic, but the reported issue is still true. In your second screenshot you show the UI Scaling menu, that's the one that is grayed out, it's on Auto, you can't change it to other values.
But, just going 100 or 200 wouldn't be that useful I think. Also, macOS has an Open in Low Resolution option, which is in some ways similar to using the scale value in Windows. So, perhaps Adobe think that UI Scaling isn't needed on Mac.
Yes the problem was resolved that Adobe Photoshop preference should have been removed it is not used any more, Windows 10 Scaling setting is used on Windows 10 with CC 2018 19.1. UI scaling was fixed with new Windows 10 features the Adobe and Microsoft worked out. UI scaling may still have issues for all I know on older Windows versions. I use Windows 10 and Photoshop scaling is atomically use and uses eact display windows scaling setting .