^ Prior to this, they never confirmed it. Did they. Pakistan had the resources to figure out if Kasab was real or not and from Durrani it was clear that they knew it long time ago. But they did not want to confirm it.
Besides Kasab wrote a letter to the Pakistan embessy in India for councilor access. Pakistan would have come clean on this if at that time they accepted him to be a Pakistani national. Pakistans credibility is at all time low now.
If Pakistan was really intent in helping India they would have accepted that Kasab is a Pakistani and then requested for joint probe into mumbai attack. The onus was on Pakistan to come clean. It was India who was wronged not the other way round.