It looks like the 300LoC simply wrap around ultralight and add a few boilerplate pieces. All the hard work of making "a lightweight alternative to Electron" is done by Ultralight, not whatever this repo is.
I haven't heard of Ultralight until 5 minutes ago, but it annoys me when some developers think they can build a tiny react app project template and then claim they created react. Or in this case, add a tiny wrapper around Ultralight and then claim they created a "lightweight alternative to electron". This repo doesn't do that.
This is nothing more than a third-party (nonfree) browser engine embedded in a basic event loop, with a catchy name, and a pretty logo.
A more accurate description would be that these are Go bindings for Ultralight.