Yes, the creator can. This category is for help with the itch.io site. Please ask the creator.
That said, it's a common problem that applies to most games. The error message means exactly what it says: the game is not signed with a developer certificate approved by Apple, so the operating system is warning you to be cautious.
I think a known workaround is to download and run the game through the Itch app. Hope this helps.