Everything is from scratch on SerenityOS. Ladybird takes as much of that as it can but the is not afraid to use existing tech for the bits of Serenity that cannot currently be used on Linux ( the GUi framework and networking ).
I think they'll replace Qt with their own GUI lib eventually - and they're already building their own language to replace C++ (for their uses I mean)...
for their operating system (serenityOS) they're not using any dependencies, they have their own windowing framework. for the cross platform version of the browser, they use Qt instead. but on serenity itself the browser does not use it.
They are using Qt and C++. They have chosen where to draw a line, which is fine, and that line happens to be on the other side of "Javascript Engine".