From my experience, each has its own problem. While learning emacs, you're very likely to hit a few keys by accident and suddenly have a new open buffer you don't understand; it takes some time to learn to hit c-g c-g every time something like that happens (is c-g quit? I'm pretty rusty these days).
Vim has other problems (like the insert mode), but even in vim you can do as the author says: always be in insert mode, and pretend it's just a normal editor.
Yes, "C-g" is `keyboard-quit'. And if you make some damage by hitting keys on accident, "C-h l" (aka `view-lossage') will show you the last key presses, so you can know what you've typed. "C-h k" (aka `describe-key') will tell you what command a key sequence runs.
The difference is (full disclosure, I pray at the altar of emacs now, after many years of vi), much of vim's functionality is hidden if you stay in insert mode, while in emacs, you always have everything available to you (and in many cases, it'll suggest "next time, use this shortcut").
OTOH, I pray at the altar of Vim while being into the church of Emacs ;-)
Modal editing is unbeatable. GNU Emacs + Viper + Vimpulse is the way to go.
I agree that Emacs allowing you to access commands while inserting text is nice. However, what are you doing in Insert mode? Hit Esc as soon as possible, my friend ;-)
You wrote "stay in Insert mode", therefore I thought you were making the mistake of staying in Insert mode longer than needed (if I misunderstood what you meant, please ignore my rhetorical question).
I didn't become proficient at modal editing until I followed the advice of staying in Insert mode as little as possible, just to enter short bursts of text (and then remapping the Esc key becomes a must).
I was saying something about the post I replied to, I wanted to point out that it's easier to ease into emacs without sacrificing the chance to learn about it for familiarity.
Vim has other problems (like the insert mode), but even in vim you can do as the author says: always be in insert mode, and pretend it's just a normal editor.