I do believe that the A318 are common type rated with the A319/A320s, so it's at least plausible. I'm beginning to wonder if DL is going for the world record for most diverse fleet.

I think you might be right though. The A318s are relatively young, but I'm not sure if I'd want a plane from a South American carrier that may or may not have been well-maintained. Add that to continual exposure to humidity. I think I remember reading something about that being a concern with used 777s from SE Asian carriers.