Your assessment is correct. As long as the protein intake stays sufficient, a person shouldn't lose muscle to a detriment while utilizing very low-carb nutrition. However, if they want to add muscle, the only way that can be done while still using the state of ketosis is to do cyclic carbohydrate super-compensation. Which is the 48-hour re-feeding procedure. The simple way to do this is to keep calories moderate, protein the same, lower fat intake, and keep carbohydrate intake around 10g per lb of LBM per day. For this purpose the faster absorbing the carbs the better, so theoretically the "worst" carbs work best for this. However some people have side-effects, like the insulin sleep, or rushes of energy due to high sugar intake. So I always recommend doing this on rest days, and sleeping a lot. The body will typically be more apt to refill glycogen stores and build muscle tissue as opposed to storing fat, and more precisely so if the last day of working out is designed to deplete glycogen.
But if a person isn't necessarily looking to build muscle, this procedure isn't really necessary. If the goal is to be as lean and strong as possible, that can be done without super-compensation.