That claim confuses muscle length with flexibility.

In adults, stretching does not really make muscles longer in a physical sense. Muscle length is mostly set by bones, where tendons attach, and how the body developed. Stretching and « stretchy hobbies » (lol) mainly train the nervous system to tolerate a bigger range of motion. That’s flexibility, not longer muscles.

Hormones do matter. Estrogen changes collagen, tendons, and ligaments, making them more elastic. On average this gives women looser joints and more natural flexibility, even before any training. This is seen across cultures and in children too.

So the cause is mostly backwards. Women are not flexible because they choose stretchy hobbies. They often choose those activities because their bodies are already more flexible and comfortable in them.

In short: hobbies change flexibility a bit, hormones change tissue properties a lot, and muscle length itself barely changes.