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I am not sure I would pigeon-hole or attach so much gravitas to the capacity to train alone.
And for the record, I do train alone all the time. I pride myself on being consistent, adaptable, motivated and self-aware. I am not the strongest guy. I am not the most gifted. But I enjoy moving. And I'll find a way to challenge myself.
But there are some people who enjoy training through play, competition, gamification, socializing and just lifting with someone.
Every single day. I'll see a newbie look at a more experienced hand doing pistol squats and pull ups and try a little harder. Or work with a little more vigor. That's what environment and people in it can do for you.
And the training, lifting weights and different outcomes doesn't come naturally to everyone. It can be intimidating for some. Confusing for some. And they don't understand the value of it. But when they do it with peers, it turns into fun or bonding.
The resilience, discipline, drive and motivation we associate with training and achieving aesthetic or strength outcomes is admirable. But you can show the same traits in say learning to do things you struggle with.
I actively avoid singing, dancing, learning languages, socializing, meeting new people (socially) and art. Things I have zero capacity for. If I truly had to be uncomfortable, I would have to embrace those things.
Training alone or sitting alone at home all day for weeks or just walking by myself quietly takes no effort whatsoever.
You need to train hard. It helps to make it enjoyable. And it helps to find an environment that constantly challenges you to do better or harder things. That often means surrounding yourself with people. Nothing wrong with that.
The self-awareness, resilience and know thyself can happen outside the gym for many.