Having no desire to grow 6 feet of hair, I’m not sure these are genes I would covet anyway, but you are correct: the “lifespan” of any single hair is determined genetically, so no amount of careful grooming would enable most of us to grow hair this long.
My older sister had knee-length hair when she was in grade school (has worn it short since then). I didn’t get a haircut for some 17 years, but my hair didn’t get past waist length and it reached that in maybe 5 years or less. There’s a point where the total length of hair that grows in a day equals the total length that falls out in a day, and there the length of one’s hair apparently stops.
When I noticed that my hair was thinning, I got it cut. Didn’t want to be one of those guys with four or five very long gray hairs.