You parsed it incorrectly. It is the other way, in fact.
Several times, people do very interesting personal projects, but fail to perform at the same level at work. That gives a clue that it may have been an unsuitable work environment that impeded their performance. It could also be a difference pertaining to their orientation to structured vs. unstructured working conditions. It could be related to explicit objectives with tight deadlines vs. exploratory development with open deadlines. And, more.
Knowing what my work environment is, I usually could understand their medium-to-long term fitness.
When I interview people, I ask them to describe one of their difficult work projects. I also ask them if they ever developed anything just for fun. I take their responses to both the questions into account when making a decision on taking them to the next round.