They generate a statistically appropriate token based on a very small context window. And they are slightly nerfed not to reproduce everything verbatim because that would bring all sorts of lawsuits.
Of course they are not reproducing Webkit or Blink or Firefox verbatim. However, it's not an "independent implementation". That's why it's "stringing together a bunch of open-source components": >>46649586
Edit: also, this "independent implementation" cannot be compiled by their own CI and doesn't work, apparently.