It's the correct meaning, as indicated by the solution on p. 430. You're thinking not of -B but of the inverse of B, which is denoted B^-1 ("B to the power of -

It's the correct meaning, as indicated by the solution on p. 430. You're thinking not of -B but of the inverse of B, which is denoted B^-1 ("B to the power of -1") and when multiplied by B gives I.