Have you read the story? It gives input to understand how this card could fit into Duskmourn.

Probably there are ability triggers like “when a door unlocks”. The reminder could be there to make clear that these effects also trigger when a room is cast and that it does not enter already unlocked.

This seems like the most likely case. I could have sworn that I saw mentions to Arena elsewhere a few days ago, but now I can’t find anything else than this wiki page.

And as far as I understand all previous starter kits did have such codes (this is my first one, so I can’t speak from experience 😅), so a copy-paste error makes sense.