"Rapid growth" is just a marketing gimmick to buy, just like "2in1" shampoo, the promised growth is unlikely to get as well as quality.
I remembered the anecdote when a man soaped the wardals with shampoo " Length and strength ", hoping for an effect)))
I can't answer for sure, because I don't know what kind of shampoo is there and what kind of ingredients it contains. But! I can suggest a method for testing at home. Wash your hair with this shampoo. It is quite difficult to see the acceleration of hair growth on the head, therefore, if lush, silky and shiny hair begins to grow on your hands and palms, then perhaps the answer is yes. So all the same, hair on the legs will grow faster. You can safely use it to grow an enviable head of hair and surprise passers-by with fashionable, summer hairstyles on your shins.
P.S. if hair begins to grow on the palms, but not lush and silky, then it's not about shampoo.
(if someone didn't understand, it was a joke. YURIKO SAN already answered)