in Mentzers Heavy Duty Journal he talks out when he was on his diet of 800 while taking deca and dbol that his bodyweight went up a few pounds while his bodyfat was slashed so he therefore gained a good amount of lean dry muscle tissue while losing fat and water

no, he gained a good amount of hormonally retained water while losing fat.
you can gain 10lbs of scale weight and lose 5lbs of fat at the same time. all water, doesnt mean theres any muscle gain.
you can also lose 10lbs of scale weight and actually gain bodyfat at the same time. try eating a
huge surplus on a strict ketogenic diet, youll drop a couple lbs of water because muscle glycogen and water gets depleted so that you LOSE weight, however at the same time you might put on straight fat because of a huge calorie surplus.
thats where the retard myth comes from that you can eat whatever you want on a keto diet and still lose weight. yeah, water, eat 5lbs of bacon a day for a week and youll lose weight at first while getting fat as fuck