According to Wikipedia, the rule for the ISBN-10 check digit is:
... that the sum of all the ten digits, each multiplied by its (integer) weight, descending from 10 to 1, is a multiple of 11.
That means, your check digit is the number which has to be added to the check sum so it is divisible by 11. When it's 0, you don't have to add anything, because that means the number is already divisible by 11.
Maybe the statement should be updated to reflect this.