Does apple reviews apps on slow network?
I don't believe that the majority of app reviews would enable Network Link Conditioner or test the app on an intentionally hobbled network.

Given the extra time that would add to the review cycle and how much time it takes for the mandatory issues, I believe that Apple's role isn't a performance tester and would expect user reviews and the developer to test adequately for network errors.

Yes - I would bet that some of the best testers test for no network condition from time to time when their intuition shows that the app is marketed in that respect or if they are looking for a reason to push a build back to the developer, but my hunch is that this isn't the typical testing workflow at present.

I think it's awesome that they noticed your current incarnation of server side content deployment resulted in a delay to the user. Did they ask you to look at using their solution for App Slicing and on-demand resources so that your apps not only installed quickly, but were useful at first launch and then could retrieve assets from Apple's CDN rather than from your servers directly?