There is some wisdom here. It's analogous to muscle memory: if you begin using a muscle in a particular way, it makes it easier to continue to use it in that way. Runners keep running, baseball players develop a personal swing and, though it sounds corny, people who smile at first eventually smile more often.
The problem is that this maxim is of little use all by itself. It's become a sound byte, a cliche, an adage easily thrown around but rarely explained.
If you change nothing but your ability to pretend, all you'll get good at is pretending. If you change your attempts to actually do something, you'll do that thing even better. That's what they're trying to say. That you should take the steps required to become successful even if you're not becoming successful yet.
This saying has more in common with "try, try again" than it does with "dress for the job you want." Problem is, almost no one explains it that way.
