The difference in data models and APIs, combined with the amount of features Apollo supported would likely make it a fairly large task.

I'm aware, but there have been efforts to do this, which is what I was referring to:

https://github.com/derivator/tafkars/tree/main/tafkars-lemmy

But that aside, he mentioned in an article recently that he doesn’t have an appetite to pivot the app towards Lemmy or another third party

aww that sucks :-/ maybe he should just put the code on github and release it as FOSS then if he doesn't want to work on it