There is no evidence of that coming from this post. The work was highly directly by an extremely skilled engineer. As he points out, it was small chunks. What chunks and in what order were his decision.
Is AI re-writing those chunks much faster than he could. Yes. Very much so. Is it doing it better? Probably not. So, it is mostly just faster when you are very specific about what it should do. In other words, it is not a competitor. It is a tool.
And the entire thing was constrained by a massive test suite. AI did not write that. It does not even understand why those tests are the way they are.
This is a long way from "AI, write me a JavaScript engine".