Looks can be deceiving... but I have been rowing a lot this summer 
Then again, didn't someone here claim recently that developed forearm muscles are a sign of faulty piano technique?
Who cares. I know Cortot method teaches you to use the tiny fingers in your hand, as opposed to the forearm muscles. There is a very fine pinpoint where you use a specific angle of your brain in your butt, and you fart just loud enough for everyone to hear...
Point is, it's not just about the forearm, or the wrist, or the finger muscles as small as my brain. It's a whole combination. It certainly helps to have the forearm muscles when you need to suddenly twist your wrist at a 110 degree angle, because it's the only way YOU can make the sound you want, because every person is different, but we are all going for that same sound. That perfect tone.