Confused about muscle gain rate

Let’s say a hypothetical trainee is able to gain 1lb of skeletal muscle mass in a month. If that trainee eats in a caloric surplus that results in a gain of 1lb of bodyweight per month, would they hypothetically be able to gain 1lb of skeletal muscle without any measurable gain of fat mass? Or is gaining fat mass inevitable and thus this trainee would need to gain more than 1lb of bodyweight in order to gain the 1lb or skeletal muscle?

No, fat mass is going to increase as well to some degree. To gain a pound of muscle, the vast majority of people in the vast majority of situations will have to gain more than 1 pound of weight total.