They have, to summarize:
A unified app would make some things easier, but it’s not so confusing IMHO. The first sentence of the descriptions in the Play Store:
Remote:
About this app: Roon Remote controls your Roon Core to play music on devices around your home
ARC:
About this app: Access your Roon music library directly from your phone, anywhere you go.
People will always be confused about something. With a unified app someone will be confused by having to use the same app for different things