Cut set means, u cut an edge or more than one edge from the graph , and graph becomes disconnected
Like bridge is very good example of cut set. In a tree every edge is a cut set, because, if u delete 1 edge from the tree, then that vertices becomes disconnected.
Now when u taking superset, means for example, u are cutting 2 edges of the graph , an then graph becomes disconnected. And u r asking is it not a condition for disconnection?
Yes, obviously it is a disconnection, but it violates the definition of cutset (which tells u cut an edge from the graph which makes graph disconnected)
If u cut 2 or more edges and then graph becomes disconnected, then it could be a case, that it is not a tree.
That is why superset shouldnot take consideration of it