[patrickambron]

He's talking about the "jack of all trades, master of none" type person.

Just because you are the absolute best at one thing does not mean you are incompetent in all other areas.

To take your example: of course you wouldn't want to promote someone who only knows the code of one project very well and of course you want to promote someone who understands how all the parts of the product work together. However, the person who knows how all the parts work together and is the absolute best at shipping code, get a lot farther than the person who knows how they all work together, but doesn't execute any given function well enough to handle on his own