Why would they be incompatible? If you're writing an adblocker for example, what you're doing is creating a blacklist and/or injecting a script. These should be engine-agnostic operations, as long as the wrapper makes the appropriate API calls. Adobe plugins in iMovie is like Chrome extensions in Firefox – this isn't necessarily possible even on desktop (it definitely didn't used to be before Mozilla changed Firefox's extension API to be more like Chrome's). But we're not talking about different wrappers, instead different API calls underneath.
Or maybe that's the part they won't bother with unless they can advertise 100% compatibility with every type of extension imaginable.