Apple frequently introduces new APIs that enable new functionality or make things easier and more maintainable. Also, testing software on older operating systems creates additional work. Unlike other OSes, macOS puts several limitations on running it under a VM, so in some cases you have to boot the OS up on real hardware. If you don't have 5 or 6 old Macs laying around this means setting up multi boot. There are also limitations on which versions the latest Xcode can target.
I understand. In this particular case, I am making an assumption that they aren't really using any new OS API's (this is, after all, a reasonably simple app) in which case they could just build and test on macOS Mojave or Catalina (the minimum OS version) and the latest macOS Whatever (the maximum OS version, for the APP store). As I highlighted, most older apps run without issue on newer macOS versions, and so that is a reasonable assumption to make and forego testing on all versions. Then, only if you get some bug report on some particular version, you can test for that ... (as you pointed out, the best approach - testing on all versions - can be very resource intensive, and this can be a practical middle-way approach for an independent developer).