There’s a lot of systems behind the scenes that we’re not privy to that occasionally bleed into the game. For instance, on this character I briefly had “The Burning Crusade” faction for a day and a separate dracthyr “battle gear” set that was just the onyx version of the starter set, but unavailable for actual use in transmogrification. The opposite faction’s reputations are actually part of our character, but set to max “hostile”. Weapon skills are still in the game but hidden; when you emerge from The Wandering Isle you can briefly see in chatlog that all your weapon skills are maxed to current level.
My guess would be that the “adventurer” class is one of these hidden systems that is applied to all player characters, but not NPCs, so the weapons or armour without that tag won’t show up for the designers when they’re adding NPCs.
Just looking at it again I think it’s a placeholder given that it’s a weapon as a quest reward in legion content where all the other classes have artifact weapons.
Edit: Begs the question on whether they’re designing the next class (if there is one) or not as we speak for the future.
If you checked the Evoker-only staff in the new raid, vault of the incarnates, last week it showed that it was only usable by evokers and adventurer.
I believe it’s the base template for characters, and they use it for testing, like billyblue said.