Most of it. The main changes from WebKit to blink are a different approach to memory allocation (oilpan) and object lifetimes to help security.

Most of the main files still say Copyright 1999 **@kde.org showing the origin of the original codebase that hasn't changed much to today.

It's now so huge and complex that a major redesign/rewrite is pretty much out of the question.

It's still mostly single threaded and all the main objects aren't thread safe. JavaScript shares that thread too. Parts of the rendering are multiththreaded and async, but that's it.