Is someone who was/is severely abused (or, may I add with respect, those who are considered mentally handicapped/disabled) not considered a person? They may act and behave differently and never live a "normal" life, but does that make them considered any less of a person?
Of course, this question assumes anthropomorphic thinking.
As for me, I believe they are true chickens, even if they were put into an environment that was not conducive to learning what we consider proper chicken behaviors. At their core, they are chickens.
Could they learn true chicken behaviors? I don't believe they all have the ability to fully overcome what they learned. You are always going to see some of those traits. Some will be better than others and "recover" better, but on average, I don't personally believe they can learn a whole new set of skills. Why? I couldn't tell you. Genetics? Because they are animals and not "sophisticated" humans? Some other reason? A combination of all the above? I'm not sure.