For "conflict-free sync," according to the docs, this appears to be last-write-wins along with a special case for deletes: https://realm.io/docs/realm-object-server/#conflict-resoluti... . (Incidentally Couch/Pouch also has a special case for deletes, but it does the opposite behavior by default – deletions lose to updates.)
There's also some language in here about how "insertions in lists are ordered by time;" I'd be curious to know how they coordinate clock times across nodes. Ditto for "last update wins" – important to know how "last" is determined.
In general, I tend to be skeptical of systems that hinge on last-write-wins, but LWW is definitely a model that works in a lot of cases where you're not too concerned about the possibility of losing data, and it's also a simpler mental model for the app developer.