As someone that had experience on both sides, AndroidX is roughly functions as a middleware that exposes roughly (but not always) the same set of API from the frameworks. Nobody sane would use the bare system framework as it’s a nightmare for backward compatibility and for handling device capability.

You’d still use the SDK the compile against, but AndroidX provides a nice wrapper that handles the aforementioned issue. It’s a separate library and not tied to the SDK.

I was using Slide for Reddit but the app wasn’t being updated for couple of months and things started to broke up.

Tried a lot of other clients and sync fits right in for me, the beta at that time was even better.