I have often been through stages where I have trained for long periods by myself. At the end of these periods I have often thought that I have made a real break through in strength only to realize that this is not at all true when I am completely schooled on going out with someone else. I hadn't realized until reading this post that it is quite a common misconception in general and is called the Dunning-Kruger effect.
So the learning is training in isolation is dangerous if you benchmark yourself mentally against what you think is good performance. Malcolm Smith seems to have been fairly successful in training in isolation so it is definitely possible but just be careful not to over estimate the effect.