Ransom from Satan is not Penal Substitution
A view of the atonement that was particularly popular from about the fourth century AD through to the eleventh was a view called The Ransom Model or Christus Victor, or Ransom From Satan. This view came in a large number of variations, but what they all had in common was that they saw the problem that humanity had as having to do with the power of Satan over humanity. Satan was seen as having powers detrimental to humanity such as control of souls in the afterlife, control of the world in this life, the ability to influencing men to vice etc. Christ was thus conceived of somehow defeating Satan and the evil powers and spirits and Death, whether through force, trickery or by his very nature. Almost no two writers agree in the precise details, but some form of this view is present in most writers from 313-1100AD (when Anselm attacked the form of the Ransom Model popular in his time).
Obviously, the Ransom model is not penal substitution, and it is often contrasted with it as an alternative model of the atonement, or they can be both held together to produce a more "complete" picture of what Christ achieved. In the Ransom model, humanity's problem is the devil, in PS it is God and his justice. In the Ransom model Christ defeats the devil and breaks his hold on mankind, whereas in PS Christ exhausts God's wrath toward us on our behalf. In some ways the two could not be more different.
Yet they can have some strong similarities. In some forms of the Ransom model it is our sin which puts us in the devil's power. In some forms of it Christ offers himself to the devil instead of us as a substitute. In some forms Christ suffers our punishment from the devil on our behalf. Thus advocates of the Ransom model can and do sometimes say that Christ suffers a punishment we deserved for our sin on our behalf as a substitute to free us from that punishment. That looks very similar to things advocates of Penal Substitution sometimes say.
There are hence strong similarities and strong differences between Ransom and Penal Substitution, as you would expect to see from two different models proposed independently as explanations for the same biblical data. But it would obviously be extremely sloppy thinking to confuse the two models, as they are ultimately quite different in what they affirm theologically. The Ransom from Satan model is not equivalent to Penal Substitution. If an advocate of Ransom from Satan should say Christ suffered our punishment as a substitute, which they are perfectly entitled to say under their view, then they are not somehow suddenly teaching Penal Substitution rather than Ransom from Satan. It would be simply a total mistake of reading comprehension to take passages from authors teaching Ransom from Satan and say they taught penal substitution.
Guess what the writers of Pierced For Our Transgressions do? They misclassify several writers in the period 313-1100AD who teach Ransom as teaching PS on the grounds of quotes that talk about Christ as taking a substitutionary punishment on our behalf. The writers of PFOT seize on these quotes and say look, they taught PS. One of their quotes even has the exact phrase "ransom... from the authority death" in it (Gelasius). If they'd even read the 'book' (ie section of the work) that their quote from Eusebius is from (Dem. Evan. 10) they'd know it's about Ransom from Satan not PS. Chrysostom, Ambrose and Augustine also are all well-known for their Ransom from Satan views. The apparently ability of the PFOT writers to misconstrue writers teaching Ransom from Satan model as teaching penal substitution is very unhelpful.