The probability that you have heard this saying is pretty high. It caught steam and it has stuck around for a long time.
If you have known me long enough we will have already had this conversation and I would have put this to rest already. But many of you are still getting to know me so we might not have had the ab talk just yet. I will give my opinion but I will also give scientific evidence will also show that it is only true to an extent.
You won't see any visual results from spending 30 minutes a day on abs when you're at 18% body fat. You can do all the abdominal work in the world, but if you're carrying too much fat, you won't see your abs.
I learned from experience early in my career that it was important to train my core often. It is nice to have defined abs when body fat is low enough for them to show. But it is also important to have a strong core for performance purposes. I saw that the stronger my core got, the stronger I was in general. I could stand for longer periods of time, perform strength lifts with heavier loads, and my lower back never hurt.
People define the word "core" incorrectly. It's not just your deep spinal stabilizers, abs, and lower back. It's your entire torso minus your arms and legs.
In 1982, the core of the human body was defined as the muscles that keep the trunk and neck in a tube like form. When your core is firm and rigid, you can do the activities it's intended to do. If the rigidity is enhanced, then you can maximize your athletic performance. This means that the shoulders, chest, glutes, abs, mid-back, and lats are core muscles.
A few studies have taken the position that multi-joint, free-weight exercises such as barbell squats and deadlifts activate "core" muscles better than isolation core exercises.
This led to the belief you shouldn't perform exercises that focus on strengthening your abs and obliques.
The truth is the common claim that "multi-joint movements like squats and deadlifts are all you need to strengthen your abs and obliques," doesn't make sense. In fact, the common push-up activates the abs and obliques more than squats or deadlifts! That is a fact.
In theory it's true that the big basics will strengthen the core. But that's only true if your core is functional and if it's not a weak link to start with. But if you can't use your abs properly then they won't receive much stimulation from the big lifts. This is why I start out building the core right away. I'm sure you have seen many "Country Core" workouts and most workouts begin with some type of core movement.
Balance. I tend to use this word a lot. But it holds so much truth when it comes to this topic. A good balance of training your abs in the most efficient manner and coupling it with proper nutrition is key. Do not get caught up in the hype.