I think the "true myofibrillar tissue" theory is the most plausible. The theory that it's best built by training with heavy weights also sounds reasonable. One thing's for sure: you can't achieve this look with any training, diet, or any AAS in five years; it takes decades of lifting. Just look at the most talented athletes of the younger generation, like Nihat Khaya. These are "young" muscles. On the other hand, muscles in a 70-year-old also look different, so we can assume that the best age to achieve this maturity is 40-50.
And if judges tell a young athlete that she lacks "maturity," that's unusual feedback. It's like saying, in other words, "train for at least 10 more years and we'll see how you look."
Player photo by
@luki7788, a perfect example of mature muscles.