This is hella necroposting but figured I'd note, Windows Defender also works off of simply assuming software it hasn't seen much is suspicious. So it could be as simple as this program hasn't been used enough to make a blip on the Microsoft servers as legit yet.
That said, this doesn't surface as a trojan but just a dialog you gotta more info your way to running. So, probably not what's going on here.
Viewing post in Bosca Ceoil: The Blue Album comments
Yes, there is also the SmartScreen system on Windows, which this app, and many other projects published on itch.io, will trigger.
SmartScreen does have a reputation system, but it is also more distrustful of an executable that has not been digitally signed. Which Bosca Ceoil Blue isn’t — because it costs a pretty penny (way more than for macOS), and isn’t generally necessary for distribution (you can just click it away, as you say).
And if you get Bosca Ceoil Blue through Steam, SmartScreen will never show up, because Steam circumvents that system entirely. Not saying that anyone necessarily should, it’s just an interesting factoid.