The Bracer of Flying Daggers (from Waterdeep: Dragon Heist) lets you use an action to activate the item to make two normal thrown dagger attacks. There's no limit to the number of times the item can be activated, though it does require attunement.
A thief can, at 3rd level, use their Cunning Action feature to take the Use an Object action as a bonus action, allowing them to activate items and do something else, or activate an item with both their action and their bonus action.
It seems like my third level thief rogue who just came into possession of the bracers is now able to throw 4 daggers per round and apply their +4 Dex mod to each of those attacks, instead of 2 daggers per round with their Dex mod only on the first attack.
However, that seems wrong, because even a 19th level fighter ordinarily gets only 3 attacks with their ability modifier (though, obviously, they get more if they action surge) and I can't imagine I'm supposed to have more than that at level 3. Moreover, this would boost the offensive benefits of the thief archetype far beyond any other rogue archetype and when I picked 'thief' I definitely at least thought I was trading away offensive potential from the Assassin in exchange for extra utility stuff. I know the thief's upgrade can be pretty crazy with e.g. a wand of magic missiles but those and all similar items I was aware of are either charged or consumable and so don't let me just keep swinging away with twice the expected damage output of a magic item round after round.
I don't actually have a copy of the source book for the item at present, so I can't second guess my GMs explanation of the item, but I am almost certain they haven't thought about the interaction with the thief archetype and are expecting a much-more-reasonable 2 attacks per round plus bonus action to hide.
How is this supposed to work? Does this really let me do that much damage?