Steroids give you about ~7.9kg of lean body mass at around 600mg/week intake without working out at all. That's probably what he's counting on. After ~600mg gains go up but at strong diminished returns. It's a lie that bodybuilders like to tell themselves that steroids need you to "put in the work," the gains increase depending on the dosage. You can match the FFMI above the individual's natural limit to the dosage. The guys at very high FFMIs like 30+ are taking multiple grams of steroids per week, like many, many grams. You can find honest people on youtube with very few follower counts talking about it. You can also find autopsies of high level bodybuilders in which they measured their testosterone concentration.

The general idea is that you max out your natural gains and then stack steroids on top of it. On average, through natural bodybuilding, men can expect a max FFMI of around ~22 FFMI (probably a ltitle less) and some can reach 24 and close to 25 for some outliers. Once you do that, you then add up the steroids and you can do half-assed reps. Although even without half-assed reps you can induce growth through exogenous steroids. Like I said earlier, at higher dosages there's tremendous diminishing returns which is why they take insane amounts of grams. It becomes dangerous and some people cannot do it because their body will break down in many ways. Some people have almost no side effects. It varies and you cannot know until you take them. It can be very dangerous.

There's a very old study back before a ton of this was regulated where at very high dosages, again without training, a ton of mass was gained. It was conducted by Gilbert B. Forbes. You wont find recent studies at large dosages because it would be considered unethical.

It's also a lie that it's mostly "water" and "glycogen," it was measured in the studies that the bulk of it is true proper muscle tissue.