The absolute newest Mac in my home is a 2017 and is limited to 13.7.8, also. It's still a beast, and I've never really thought of it as "old." The macOS (and iOS) ecosystem, though, is brutal on us "slightly older" hardware owners. We get dropped so quickly, by both Apple and by 3rd party developers.
Windows developers would think nothing of keeping their applications running on Windows 7 (16 years old) or Windows 10 (11 years old), but my 9 year old Mac is somehow ancient.
Subtle bugs always find their way in increasing amounts for Windows applications that continue getting software releases; we tend not notice because we all run actually supported versions most of the time, and even when we dont- its only for a year.
I see people on youtube trying to make “modern desktop” experiences on Windows 7 and 8; and it takes some serious doing with all the incompatibility with things like browsers. Dialogues about missing features crashing you to desktop more often than working.
So much so that there are dedicated forks of chrome and firefox to support this purpose.
not the design per se (however you are right that theres a lot of swiftui usage here that is only available on newer macos’) but mainly because it is using the new @Observable observation macro that is only available on macOS 14+