One hit in four takes one soup from your opponent's inventory and puts it in yours. At most one theft every 4 seconds, whatever your click speed.
The point is not the soup gained: it is the soup lost by the other player. Over a long fight, the Baku turns a war of attrition into a material advantage, without ever hitting harder.
Past 24 soups on you, the stolen soup is destroyed instead of taken. You still deprive your victim, but you never go beyond what a kit gives you at the start — the Baku saves trips to the refill, it does not replace them.
The counter: it does not finish fights, it sets them up. Against someone who kills quickly, it never has time to build up.