I find it hard to imagine that an industry veteran like Carmack would write code for another company while employed with another and proving some bit of proprietary advice was incorporated into the product would be difficult.

Back in the day id programmers shared snippets of code with whoever they wanted - they didn't need to ask any corporate overseer for permission. There were a few little bits found when the GarageGames/Tribes engine code was first released, for example. (pretty sure they got those lines of code from Brian Hook, but it's been a long time...)