Good question. First, remember that, while we use the New American, it’s commentaries are oftentimes not in accordance with what the church actually teaches. The authors seemed to be doing their own theology in some places.
That said, remember that the church, while defining purgatory, has never actually said if it is a location (which it probably isn’t since we exist in a spiritual state until the last judgement), a real time period, or a process. I agree that 1 Cor. seems to be talking about judgement, but it gives the fate of three people. First, it talks about those who build their foundation on Christ. Those who don’t, obviously, would go to hell. Secondly, it talks about those who build of gold, silver, and such and says they are saved. They, obviously, go to heaven. Lastly, it talks about those who are saved but suffer loss. Since we aren’t saved in hell and we don’t suffer loss in heaven, only one possibility exists, which is purgatory. It could very well happen instantly, which makes it a matter corresponding with judgment. Remembering that we will exist outside of time, your protestant friends are incorrect in trying to disprove it through chronology.