It didn't originate with Joel Spolsky. "Eating your own dogfood" was already a meme at Microsoft when Spolsky arrived there (1991), and that's where he probably picked it up from. Wikipedia attributes it to Paul Maritz:
"In 1988, Microsoft manager Paul Maritz sent Brian Valentine, test manager for Microsoft LAN Manager, an email titled 'Eating our own Dogfood', challenging him to increase internal usage of the company's product. From there, the usage of the term spread through the company."
"In 1988, Microsoft manager Paul Maritz sent Brian Valentine, test manager for Microsoft LAN Manager, an email titled 'Eating our own Dogfood', challenging him to increase internal usage of the company's product. From there, the usage of the term spread through the company."
https://en.wikipedia.org/wiki/Eating_your_own_dog_food#Origi...