With AMD's Infinity Fabric, I wonder if they can slap a Ryzen CPU die on there, as well as multiple Navi GPU die's? As of now, it seems that they can use it to tie multiple CPU's together, or multiple GPU's together, but I haven't seen anything talking about a hybrid design with both just yet. That way you wouldn't have an APU, but you also wouldn't have a separate discrete CPU and GPU. It would basically be a hybrid, so someone could say it's not an APU, but they wouldn't be saying it's definitely discrete either.
If PS wants another large leap around 2020, planning to use an APU will make that much tougher if not impossible depending on the node available at that time. If they want a large jump, they would need to go discrete as far as we know, or come up with something else, like a hybrid, if it can be done.