Two unrelated questions. Native libraries would — at least — make the application faster, and considering this is an image-related program I don't want to imagine how slow it is being rendered via a hard webview instead of a light one powered by a native framework like Cocoa. On the other hand, if it is difficult to implement an scrolling list of images and a toggle button using either Objc or Swift, that is something I don't know because I have limited experience with those languages, but if you take the time to build a non-cross-platform application, why not use native libraries?
If they are planning to package this program for Windows, Linux, Others then good, nice release, but I would still criticize because they are using a technology that is supposed to be a tool to build cross-platform applications from the beginning, if they already have the HTML / JavaScript code for this, what is stopping them from packaging the Windows version for example? I don't see anything in the feature list that requires Mac specific hardware / tools.