The author is ignoring that:

A) Most sites do have content with which async GA can load in parallel.

B) Where you put the GA snippet is a trade-off. Putting after window.onload will decrease the time it takes to get to window.onload, but it will also lessen the accuracy of your data; especially your bounce rate.

C) If you're worried about delaying user visible functionality that is tied to window.onload, you shouldn't be tying that functionality to window.onload in the first place. There are better ways to detect when the DOM is ready to use.