AFAIR, the early Fediverse crowd mostly used Linux. They didn't have a problem with choice. Then again, they generally didn't even have a problem with self-hosting.

But I think another issue with iOS in particular is that it's rather difficult to get Free Software into that ecosystem as long as the Apple App Store is the only way to get apps onto your device. It's an issue because Fediverse app developers seem to generally prefer to put their code under free licenses, especially the GPL.