As was the case with the Borderlands games, spin-off Tiny Tina's Wonderlands drops randomized weaponry in an endless gush, like a fountain but with sniper rifles instead of water. Those guns can only be used if you're the right level for them, and it's common for a quest to hand out something you won't be able to wield until the next level-up. What's not common is for that level to be higher than what's currently achievable in the game.
As afrankiewicz12 on the Wonderlands subreddit spotted and commenters confirmed, you can receive weapons as high as level 43 (in this case a legendary submachine gun that shoots fire bullets and does more damage after you cast a spell). It's not possible to actually get to level 43, however. The level cap in Tiny Tina's Wonderlands is 40, and you'll probably finish the main storyline around level 30 or so.
Hang on to any level 41+ weapons you find, though. With previous Borderlands games the level cap has gone up with the release of expansions and patches, and the first part of the season pass for Tiny Tina's Wonderlands is scheduled to come out on April 21. Presumably it's just an oversight or bug allowing non-wieldable weapons to drop right now, and given time we'll be able to hit a high enough level to shoot that legendary firebullet SMG, and anything else above the level cap that gets handed out.
Thanks to GamesRadar for the tip.