I agree that (credit - debt) = zero. The problem is that credit is not a fixed amount due to the fact that it is leveraged... see fractional reserve banking. http://en.wikipedia.org/wiki/Fractional-reserve_banking
The leverage formula is (1 / reserve) where reserve is a fraction tending to zero. This is a non-linear relationship and we all know that the lim x -> 0 (1/x) = infinity.
If lenders (e.g. banks) hold a 20% reserve, they will "expand" the money supply by 1/0.2 or 5x. If they hold a 10% reserve, they will "expand" the money supply by 1/0.1 or 10x. If they hold a 1% reserve, they will "expand" the money supply by 100x. As the reserve fraction goes to zero, the effective credit part of Martin Wolf's equation goes to infinity.
A lot of exactly that type of lending was being practiced during the "bubble", especially in the housing market where mortgage underwriters were loaning more money than the houses were worth. This was happening before the housing crash - mortgage originators were advertising "we will loan you 100% the value of your house" (sometimes more than 100%).