Even if using a framework like RN, solo devs would be putting themselves in a precarious position if they don't understand the underlying systems and frameworks. Without that knowledge they won't be able to effectively debug and figure out if issues are stemming from the cross-platform framework or the native platform/framework and subsequently will be entirely dependent on outside parties for fixes.

So if they're learning the native frameworks anyway, they may as well use them. These days with Swift and Kotlin most logic code transfers pretty cleanly and most apps can be LEGO'd together from UIKit/SwiftUI/Compose components, so the workload isn't as high as one might expect.

Aside from that, early on it's usually better to start with one platform, nail the UI and interaction, and then once the app is proven worry about the other platform.