"I never drink your coffee; it's too bitter."
Translation:Je ne bois jamais de ton café, il est trop amer.
I don't see how de can possibly be the indefinite article because articles can't be used with a modifier like ton.
My theory is that de is a preposition here, used with jamais and ton café to form an expression of quantity. So this expression (jamais de ton café) is analogous to beaucoup de ton café or plus de ton café.
This is just a hypothesis. I hope someone who actually knows will weigh in soon.