This is a mixed bag.
Yes. You can build good quality lean muscle on a keto diet. But you can also build quality muscle on a diet that incorporates carbohydrates. If you’re lean and trying to add muscle to a small frame, I suggest leaving carbohydrates in your diet.
Insulin plays a key role in shuttling certain amino acids into muscle tissue. And the extra carbohydrates help. But, amino acids generally elicit their own insulin response to a much smaller degree. But enough to do the job. Most individuals want to add muscle while staying lean. So, yes this kind of diet is effective for staying lean and adding muscle.