Yes and they implement EVERYTHING in house. In case you haven't heard they also started implementing a browser engine from scratch https://ladybird.dev/ just for fun. It kinda took off and they even got some nice donations, just to keep it going and see where it leads.
The browser was at first only available in serentyOS itself but lately is available as a stand alone program running on other OSs as well. It's still pretty early days, I am exited to see where all this leads tho!
It's a work in progress. Most sites won't work but some do. Check out this latest development update video: https://inv.tux.pizza/watch?v=giq5iXJntgQ&t=911 That link leads directly to the "demo segment" where he opens some sites.