I wonder if they used a cross platform library abstracting the two.
I'd prefer something like Fork over something like GitKraken IMHO. I cannot imagine the performance of some electron alternatives if I were to open a repository with 200k commits, but this can be resolved with shallow cloning.
Even if it has lots of Swift frameworks embedded, as soon as they switch to Swift 5 and Mojave and higher, Swift will become built in with no dylibs.
I'm curious why they didn't use Xamarin.Mac though, they could have shared a lot of code between apps.