It's worth noting that you need access to a Mac you can trust to setup your Apple developer credentials on as well as iSign and then export your credentials and certificate from each time you get a new certificate. It's likely you won't be able to do this in the Apple store. While you or I may have a colleague or trusted friend we could do this with, many developers will not.
It does say that in the readme, but I don't think it's actually necessary: you can get certificates and provisioning profiles through the web portal without going through Xcode. (For certificates, it has you upload a standard CSR file.) Though developers may not know that…
Admittedly, if you don't have a Mac build machine, (a) you can't test your builds, and (b) you can't use the official SDK (including, e.g., system header files) without violating the terms of service. Not that many people care about that, but if you don't mind ToS violations you may as well just install pirated macOS in a VM (which is easy enough in practice). Perhaps Apple deserves blame for not having a legal way to run a macOS VM on non-Apple hardware; certainly it makes life harder for open-source developers that want to play by the rules. Still, these obstacles have nothing to do with signing.
