This is a total myth. It doesn't cause permanent hair damage. Only temporary breakage at most.
It's just that when you sport a man bun your forehead looks bigger and people can clearly see your exposed, true hairline.
VVD's hairline has always been like this. Did Ronaldo's hairline look bad or anything different when he was sporting one at Juventus?
Andy Carroll, Nunez, Tsimikas all sport buns and have fantastic hairlines.
Putting pressure on your hair follicles is bad but it can only cause temporary damage/poor hair health. It will never cause permanent damage because the follicles will still be active and viable for hair growth later. Only genetics can fuck up your hair permanently. Too much of harmful chemicals can damage your roots to a reasonable extent too.