Cool story. In a way I'm surprised one would try to adapt the old code after so much time has passed, rather than just starting fresh. It can definitely be interesting, just more... frustrating.
I guess Objective-C/Cocoa might have not changed much between the last HyperEdit version and now. If the developer organized his codebase well, replacing the old webview with the new stuff and so on should be less work than rewriting the whole app (IMHO).
I haven't coded for the mac in more than a decade but what I remember from ObjC/Cocoa was that it was quite pleasant to use and very powerful. By powerful I mean that the ratio between LoC and smiles on my face was skewed towards smiling.
That's true. I maintain a couple of obj-C iOS apps (one of those ~10 years old) and apart from framework upgrades (iOS 6-7 was a big one) and dealing with a few deprecations, it's been surprisingly friction-free.
macOS is probably even more stable.
Swift, on the other hand... it's been a moving target for sure, and I'm glad I didn't just rewrite everything (like I did for one app).
I imagine in the end not much old code was left, but if his initial code was well structured I can also see how it helps to not start from a blank slate. The structure you see in the names of files, classes etc. can make it easier to keep a mental model of the app that you’re building.
I haven't coded for the mac in more than a decade but what I remember from ObjC/Cocoa was that it was quite pleasant to use and very powerful. By powerful I mean that the ratio between LoC and smiles on my face was skewed towards smiling.