A few of those under-5%ers got drafted fairly high in the NBA last week.
The first that I remember hearing of low body fat being dangerous was regarding a USC Trojan running back in the 1970s named Ricky Bell.
I had a buddy who worked in the clinic that served USC’s athletic department. It had a hydrostatic dip tank that measured body fat and IIRC Bell set the record for lowest % that they had ever recorded.
Bell went on to an NFL career, but fell ill and died very young at age 29. The reports were that his illness was connected to his extremely low percentage of body fat.