It's debt regardless of whether or not one pays interest. Debt isn't linked to interest. Just means that you have an obligation to pay money to someone.

EDIT: Though in fairness, if one never actually uses a credit card at all, then one never takes out debt, so I suppose it's probably better to say "if one has a credit card that one uses".

EDIT2: Though all this is not to diminish your point that not carrying credit card debt from month to month is generally a pretty good rule to live by.