Yes, but the engine is running in a separate process, and communicates over a generic engine protocol, so that is no reason that would force open sourcing the GUI.
Wonder if it can be still compiled for modern Apple Silicon MacOS given Cocoa have been pretty much removed
Cocoa hasn't been removed.
I had to change some .strings file encoding settings and set the build target to macOS 14, but other than that and a few deprecation warnings, it built just fine in Xcode 27.2 on Tahoe 26.7.2.