It's a myth straight up. The only thing that stunting the growth of a FH does is case health problems later on in life. Just remember just because the body itself isn't growing that doesn't mean that the organs inside don't continue to grow at a normal rate. This sadly leads to the FH having a much shorter lifespan all just for the sake of a smaller more compact body.
Honestly the best ways I've learned to improve kok growth is to allow it to have plenty of space, clean water, good quality food and after feeding I show mine a mirror every now and then to kind of amp up the aggression. Doing this will give the FH a sense of competition along with a sense of victory when the invader/mirror is taken away. Just don't show em the mirror before eating otherwise they'll usually refuse to eat and only focus on the mirror. I only use the mirror for about 15-20 mins at a time, any longer and they tend to lose interest in it, atleast ime.