You guys are both kinda right. Testosterone has some myostatin-inhibiting aspects to it, so that’s part of the reason why. Higher testosterone———> lower myostatin ———> more muscle. It’s not the whole picture, but it’s part of it.
That’s the cool thing about Captain’s myostatin-inhibiting field. Women (as well as men) can add muscle much more easily without loading up on testosterone and other hormones. A “cheat code” in a sense, as are pretty much all of his fields (in a good way).