It is really about the angle of your feet and the width of your stance, most anyone without a debilitating injury can learn to squat in a few minutes when given correct instruction.
No, you don't need to be a gymnast but there are still plenty of people for whom the angle of feet and width of stance is totally insufficient to do a squat below parallel because they lack the necessary hip flexibility.
If you can touch your toes with straight legs, you can probably squat below parallel for at least a short while - it's not like it takes a huge amount of flexibility. But that too still requires weeks of training for a surprising number of people.
It's not hard to get there, but to write it off as a myth is not helpful.
However, there's still a huge difference between getting your hip crease below your knee for a moment, and being able to sit in the bottom of a squat for a prolonged period in comfort.
I can clearly remember at school, 10 -12 years old, during winter after heavy snow, my friends all sliding downhill, feet absolutely flat on the floor, sat hunched as low as possible.
Absolutely impossible for me - I couldn't get my backside within 30-40 cm off the ground no matter what I tried.