It's negligible. Like fat people blaming "muh metabolism!" skinny fags blaming "genetics" is just a lazy fucking excuse.
Yes, there is a difference, but it really isn't noticable with the right training and diet.
You can GUARANTEE that someone who "isn't responding" to training, simply isn't eating enough. Sleep and training method might also come into it (and again, can be fixed) but 99% of the time it will be diet.
This study is interesting, and I will read it properly, but flicking through it seems to say "yeah, some people don't respond as well, but it's not a major issue". Also note in the cited study, all participant were OVER 65.
Also, like said, steroids really help. If anything they help skinny fags become able to eat more if they physically couldn't before due to years of undereating. Certainly did this for me.
Also, the "hair vs. responsiveness to steroids" thing does have a tiny bit of truth, as hair loss is as a result of conversion of test into DHT, but again, it's a negligible amount.
If you find me a guy who is eating clean and way above maintenance, training heavily with progressive overload, and getting decent sleep, but he doesn't grow, find me a scientist and I'll pay for him to be studied as he might be the key to some important genetic discovery.
If you find me a guy who does all that AND is on gear, but STILL doesn't grow, I'll cut my balls off with a chainsaw.