They did answer your question. Based off what they told you, one can reasonably assume Sync for Lemmy will be to Lemmy as Sync for Reddit was to Reddit...
It was named Reddit Sync before Reddit forced apps to rename a few years back for trade mark reasons.
Sync was named for a feature it had where you could sync posts to your device for offline use. It was super useful as recently as a few months ago when I wanted to mindlessly scroll on a plane.
Whoa, Sync has been my most used Android app for the past 5 or 6 years and I never knew the origins of its name. I just figured naming products is harder than most people think (trying to be original and memorable) hence the basic Sync moniker.