I think your review is a little bit inaccurate - the stated purpose of the book is to place penal substitution alongside the other 'stories of salvation' (as Holmes puts it), which is where it should be: one metaphor among others. How can a punishment also be a ransom and a victory over death and Satan? Only by acknowledgeing that they are all metaphors for a mystery. How can an electron be both a wave and a particle? It isn't either, but those metaphors help us understand a tiny bit about the mystery of the atom.
I suspect Holmes might even agree with Steve Chalke that when it is exalted above all the other stories, the concept of an angry God torturing his innocent son becomes a mockery of the gospel. Holmes is trying to save the idea of penal substition from those who somehow see it as a superior story to all the others, and somehow 'more true,' despite it being much less developed than other ideas in the first 1500 years of the church. By making it part of a matrix of metaphors, he makes penal substitution more credible.