This is webkit, which is open source. Apple took an existing HTML/CSS/DOM engine, rewrote it, renamed it, and opensourced its version, too.

It's compiled using LLVM, which also contains thousands of lines of open source code by Apple.

Of course you might argue that these examples don't prove your sweeping statement false, but please read https://en.wikipedia.org/wiki/No_true_Scotsman before arguing.