Sadly, you cannot copy the MP3s off an iDevice using Pocket Casts. I wanted to build an archive of a couple podcasts, and lost months of episodes when I wanted to replace my iPad.
This is probably to late for your use, but there is a neat Mac utility called iExplorer that lets you browse your iDevice's filesystem and copy files off of it. I'm pretty sure you could have grabbed the files using a trial of iExplorer.
Indeed, I have already flattened that device. however, I will re-install pocketcasts to see if I can extract stuff.