Whats harder to believe is that he has the same amount of muscle fibres in both the before and after photos.
Proof of this is that when he stops the gear and lifting he will very quickly look like he did when he started unless he gets fat as fuck.
If he gained new tissue that tissue would still be there afterwards
Nit picking here LOL, same amount number of fibers but the protein content can increase or decrease aka tissue, it's not all water, there's more meat

I may be way off here but I suspect there might be more happening here that the studies so far haven't quite caught because they haven't had subjects like this

Long-term anabolic steroid administration leads to significant changes in human skeletal muscles, including:
Increased myonuclei density: This is a measure of the number of nuclei in muscle fibers, which can persist even after stopping the use of AAS.
Enhanced muscle growth capacity: Former AAS users often show greater myonuclei density and a higher capacity for muscle growth compared to non-users.
Morphological changes: AAS use can induce sustained morphological changes in skeletal muscle, leading to improvements in muscle strength and size.
These changes suggest that AAS can have a lasting impact on skeletal muscle, potentially leading to an increased risk of muscle atrophy and other health issues [if the use is discontinued.
When reintroducing lifting or steroids you regain your previous muscle much quicker than you gained it the first time around suggesting permanent changes.