There's no doubt in my mind that Andreas could achieve that by himself. He's worked professionally on webkit, and implemented a JS interpreter, a JS bytecode interpreter, and a JS JIT all by himself after all. Also let's not forget that V8 is open-source, all their optimizations are available for others to see and implement.

But to be clear this isn't a one man project, he hired a few contributors to work full time on it. Sure, it's a small team, but as said in sibling comments a small team has much more velocity.

The theory is that JS JIT compilers don’t actually improve real world performance on the majority of websites. This was apparently per the advice of the authors of Chrome’s and Safari’s JITs.