The article that the author links in that sentence goes into more depth; it claims that async/await is better in part because V8 can optimize it better than it

The article that the author links in that sentence goes into more depth; it claims that async/await is better in part because V8 can optimize it better than it can Promises. In other words, async/await is better than Promises, provided that you are not using polyfills. He isn't assuming polyfills are free, he is assuming that you don't need them (or that you can pull them out of your bundle over time)