They’re simply mistaken, ostensibly due to a lack of understanding with regard to the real nature of graphic design work. There are some here who’d say graphic designers should not only be educated, but should also have to carry some form of license to practice.
It could depend on where you are; currently in the United States, a career in graphic design can be nearly impossible to get off the ground without a college degree. I came into design when it was made part of a seemingly unrelated job over 30 years ago, and it became a career of sorts despite my having mostly self-trained. That initial progression lasted quite a while, but when it came to an end several years back, I was unemployed for the first time in my life. But I expected I could continue my career, considering I had over 2 decades of experience. Not so. Without a degree, I struggled even to get an interview for entry-level graphic design jobs. When I did get some interest, they’d ultimately decide I was overqualified, or just offer me far too little money.
So, when anyone in America posts here pondering whether they should study formally, my answer is always the same: I’ve had a long career in graphic design without a formal education . . . you won’t. Go to school.