This app (in its current version, 2.4.0) does not seem to be available for "OS X 10.11.0 or later" as the page here says... it does not seem available for High Sierra (10.13), Mojave (10.14) or even Catalina (10.15)... it seems that someone confused macOS 10.11 with Big Sur, which is macOS 11, but is also known as macOS 10.16... the AppStore says that it requires macOS 11 (Big Sur).
BTW... I see no real reason why is such an app requiring Big Sur... if it wants to support M1 Macs, Universal binaries are supported on earlier macOS versions too... those with M1 Macs would have to use Big Sur (since you can't install earlier versions of macOS on M1 Macs), yes, but why require those with Intel Macs to upgrade to Big Sur? Even much more complex apps support High Sierra (or even Sierra and older macOS versions if they don't do something graphically intensive to need Metal 2, which became available with High Sierra) and they can just add support for M1 Macs without requiring those with Intel Macs to upgrade to Big Sur.
Most macOS user probably haven't upgraded to Big Sur and many haven't even upgraded to Catalina yet.