Knurid the Wild my Nord isn't the Dragonborn, but wants to wear heavy Stalhrim.

What leads you to think otherwise?

Far as i know, the main quest, and thus Dragonborn status, isn't required for the side quest that allows for Stalhrim gathering and crafting.

I thought you had to start the MQ of Dragonborn which requires you be Dragonborn, in order to help free the Skaal and cleanse the totems?