Isn't it rendering html5/javascript like a browser would? it would be great if somebody could take time to reply and explain things in a calm logical manner without reaching for downvotes
I think nobody wants to tell you since it’s something that is easily found on the linked site.
To answer your question, there is no more HTML involved in this than in a React Native app. The result looks like HTML, but it’s really used to compose native OS widgets.
Then this project sounds perfect for you because it's not a browser, it's Qt5 but called via APIs that JS developers are used to.