"That's true if and only if he didn't continue to refine and contribute to the code after entering employment, and specifically, didn't use it at his place of employment."

The same could be applied in reverse: if he wrote a considerable part of the code before entering employment and did use it at his place of employment, what does it say about the employer's right to use it? Or is it "since you're working here now, we can use all you ever wrote exclusively even if you didn't license it to us"?