That's technically true! Braids, weaves, wigs, hair buns are protective styles. Hair is always growing, but when you're using heat, color, chemicals, brushing/combing your hair every day. Your losing hair. Braids and other protective styles retaining hair because when you're wearing these styles your not losing hair by flat ironing, combing, etc.
When I get braids and leave them in few a few months. My hair always grows with braids in. Because I take care of them the same way if my hair wasn't braided. I still wash and oil my hair. Some of the people with the longest hair are people with dreads and naturals. There's a black woman with the longest hair and its dreads. Because it takes more effort to maintain straight hair than curly/natural hair. But there are people with long relaxed hair.
Maybe a bad analogy. But it's like a cut, if you keep picking at it takes longer to heal, than if you leave the bandage on. And let it heal on it own.
This is all truth. The biggest reason is that black people have the most delicate hair out all races. Which means that our hair can be easily broken or damaged. Therefore wearing protective hair styles helps us grow our hair.
Plus protective hair styles prevents new growth from being damaged.