Don't they maintain shadow accounts and not actually delete the account in the background. That was my understanding from prior discussions around it. Basically they hide the account, not actually delete the account and all data associated with your use.
They absolutely delete the account and data associated with it, however the shadow accounts thing is... separate. Their line of reasoning is: well, I uploaded your phone number, so it's "my" data and not "yours", so "you" can't delete it...
Could be totally wrong here though...