You could have just write the performance critical part of your Electron app using C++ and native node module and achieve the same performance improvements. You actually did it, but using Rust instead of C++.
ClipBook is written using Molybden SDK. It allows building UI using modern web frontend frameworks like React, Vue, Angular and write business logic using C++. In case of ClipBook: UI — React + shadcn/ui, business logic — C++ + Cocoa. I wouldn't say it's a native macOS app. I would call it a hybrid app that combines web and desktop technologies to build cross-platform desktop apps quickly.
As for access to the source code — I see that it's very important for lots of people, so I will open the source code on GitHub. It's not a problem at all.