This isn't the first time I've read somewhere that states that leafy greens aren't good for rabbits, and personally it seems like a very flawed conclusion to me. Alfalfa is a plant just like leafy greens. It's just usually given to rabbits in a different form, like dried for hay, or processed and compressed into pellets. But it starts out as a plant with leafy green leaves. I've personally had more trouble with alfalfa pellets causing digestive problems with my rabbits, then I've ever had with feeding them leafy greens. Leafy greens are like anything else you feed your rabbit in that you have to make sure that it agrees with your rabbits digestion and doesn't cause problems. Alfalfa and alfalfa pellets are no different.
I have a rabbit that I got way too young, and was supposedly eating rabbit food and weaned from it's mother. Well, I brought it home and it wouldn't eat hay and wouldn't eat rabbit pellets. Really long story, but what happened was that I ended up having to feed her green leaf lettuce,cilantro, and other leafy greens, for about 10 days, at which point she started nibbling on pellets and eventually eating a normal amount. I don't by any means recommend this, and almost everything says that it's not good for baby bunnies to have veggies or leafy greens til at least 12 weeks. She was much younger than that and was perfectly happy and healthy eating the greens. It did make her pee a lot though, because of all the water in the lettuce... She is now one of the healthiest rabbits I have. I never had problems with her health and she was raised eating leafy greens. On the other hand, the babies that I've raised on alfalfa pellets and hay only, I've had very frequent digestive problems with. I'm not recommending feeding young rabbits veggies, but I have my doubts about all the recommendations that veggies aren't good until they are older. It certainly hasn't been my experience that veggies cause digestive problema in young rabbits.
My opinion based on my experience, if it doesn't upset your rabbits digestion, then leafy greens are probably one of the better things for them to eat.