Yep. This has been pretty much common knowledge as long as I've been alive. As someone who lifts, it's been ingrained in my mind "abs are built in the kitchen, not the gym."

Sleep is also a commonly overlooked but incredibly important aspect. You could be busting your ass in the gym, have your diet on point, but not be making gains if you don't sleep or sleep poorly. For hypertrophy focused folks, muscle isn't built in the gym, it's built while you sleep - you tear it down in the gym so that it can rebuild during recovery.