To be fair, archdruid is a title. Not a power level. Not every grove is going to have an 20th level druid amongst them to fill the role of Archdruid. That would be insane. Especially not such a small grove that is so far out of the way in the middle of a territory that seems to mostly deal with goblins. If the most powerful druid they have is only 4th level they aren't going to just not have an archdruid until someone levels up enough first.
Not sure if the fifth edition changed something, but in D&D it used to be that level 12 was the minimum requirement to even consider taking the role of ArchDruid.
