you can make the snowball strategy be the optimal strategy by simply picking the right cost function.

optimality entirely depends upon your cost function. so pick the right one and optimise. the author clearly has one in mind---paying out the least money to eliminate the debt. but this doesn't seem the right one to me.

for example, i would much rather pay off small debts to friends (which typically have 0% interest) over larger, higher interest rates to banks, simply because there's a social cost owing money to friends, whilst it's kind of the purpose of a bank.