I don't like this one not because I don't agree with what I think the point of the advice is: don't over swing on the backswing and don't do a sort of jerk start to the down swing trying to get to 100% club head speed as early as possible. Not that anyone's really trying to do the latter, just that I sympathize with that thought because it took me a really long time to overcome the baseball instinct of as soon as you decide to swing, you max out, swing as hard/fast as you can cause you gotta get the bat to the ball before it flies by you!
My problem with this advice is that it's a weak teacher's replacement for actually teaching a better swing. If you've got swing flaws that get you too long of a back swing and with a bad position at the top, and then your down swing is all out of sequence with some parts jerking and others left behind or out of position, then sure, just swinging at 60% of your potential good swing speed will allow you to compensate for some of those things manually through the swing and get somewhat better results (i.e. more often near where you want, though shorter and still not consistently good).
But that's not actually fixing your swing or helping you improve on a long term path. It's just accepting your crappy swing and swinging slow so you can successfully compensate a bit more often.