There’s a world of difference between plugins to Apple-level services like WebKit, and allowing third party apps to run arbitrary code. And to top it off, both Adobe and those plugins would need to adopt whatever mechanism Apple will use, which by its nature will be proprietary to iOS (and maybe Mac).
Marzipan is more about UIKit on Mac than anything else, this doesn’t really fall under the purview of Marzipan in the first place. The limitation is in sand boxing and App Store guidelines, really. You can technically load plugins already on iOS, from a developer standpoint, they just have to be in your bundle to begin with. So to enable it for iOS 13 would be a separate productivity push, and not really related to the Marzipan project.
Even if Apple does it, that is an extra set of “what if’s” on top of that which makes me thing uptake would be slow, even if Apple pulled this off next year. Actual real support by plugin devs would be years away, realistically.
But since iOS 13 isn’t even in the planning stages, nothing we can speculate about even matters in what this first version will look like. Whatever Adobe is cooking up, it’s likely built on iOS 11 as a target, due to the development time required for such a port.
Keep in mind it took Affinity a decent chunk of time to get their iPad apps out the door. Designer had a very short teaser video out prior to 2015, and Photo was teased at WWDC 2016 in detail. These were both long term projects for them, and they have a codebase not quite as bound up by legacy concerns like Adobe does. There’s no way Adobe hasn’t already been working on this since Affinity Photo launched last year, if not even earlier in response to their WWDC tease in 2016.
All this adds up to me being very skeptical of them delivering anything that Affinity Photo hasn’t already in terms of functionality.