Yeah, that's... wholly incorrect. "Muscle confusion" has actually reached myth status. It's a super-short cycle bastardization of periodization (planned cycling for peak performance at competition) that companies use to sell their video tapes to consumers with the attention span of a goldfish.
Your body does not stop making gains as it gets used to a certain movement. I want you to sit down and think about how stupid that sounds. What you're saying is that sprinters don't get faster primarily by sprinting more, but by doing something else. The best way to a heavier deadlift is to deadlift more. You get a heavier benchpress by benchpressing more.
You don't shock the body into gains. Sure, you get a heck of a pump the first time you're in the gym after a week off, but that in no way means it will make you stronger... or that the size will last. If the body's shocked by anything, it's probably also pretty darn close to unnecessary injury.
Also, I'm gonna throw this out there: stop isolating muscles as your workout. That's for weak people that don't know what they're doing and professional body builders. There are some isolation movements that make good accessories to your primary workout, but you won't need them until you're a more advanced lifter.
Insofar as complimentary muscles are concerned: First off: If anything feels 100% at the gym, it means you haven't been to the gym enough. No muscle should ever feel 100%, except on competition day if you compete. Using a muscle that "60%" will still make it stronger. It's just not going to boost your ego. Muscle strength is developed from overloading, not under-loading. You should always be using complimentary muscles as a consequence of doing anything. If you leave the gym feeling fresh as a daisy, you're weak as one, too.
Now, don't take that to mean that you should just do one movement every day... unless you're doing a full body workout daily, you'll naturally vary just to hit every primary muscle.
An example of what I mean:
Day 1: Front squat, strict press, rows
Day 2: Deadlift, btn press, dips
Day 3: Cleans, overhead squats
Day 4: Squat, benchpress, rows
Lift Big Eat Big: The Muscle Confusion Myth