I used to be co-maintainer and I'm working close to full time on Guix. I also happen to have worked on Guix on Mac before; in fact I'm involved in the very mailing list thread you dug up.
We are familiar with the way Nix does it on Mac, but I won't repeat my assessment here. Doing it with XCode and adding an arbitrary cut to the graph would get us to something that resembles Guix only in uninteresting ways.