The fact that improvements hold for CART (CCP) shows that the effect of HS is not entirely replicated by tree structure regularization, and instead, the two regularization methods can be used synergistically. Indeed, applying HS can lead to the selection of a larger tree. Since tree models are sometimes used for subgroup search, larger trees from HS could allow for the discovery of otherwise undetected subgroups.
1.4 CCP and HS can be used synergistically