By "downloaded," I expect that you mean "Built, tested, and deployed." It's not an App Store app. It's basically a technology demo. Get Xcode, and build it and run it.
You could always request that from the author. Since it's a Mac app, they could do that. Not so, if it were an iOS app.
It's a pretty basic SwiftUI app. They haven't really polished it, so I could see why they might not be interested in making it much more accessible. It's a tool for Mac geeks.
Speaking for myself, I have a whole bunch of packages, and almost every one has a test harness. Many of the test harnesses are "full-fat" iOS apps, so they can't be provided as releases, unless I create an App Store app for each one.
They need to be built and run. A couple are Mac apps, but the whole deal with them, is that they are test harnesses, so divorcing them from the IDE is sort of negating their purpose. They are meant to help other Apple developers to understand and use the packages the apps are associated with.
