This rhetoric is counter-productive. Those who have served sentences, indeed anyone, can only be deported to their COO if there is evidence, acceptable to the native country that s/he is a citizen. It's in the interests of that person not to have such evidence.
They have listed 23 countries who have agreed to this.
No, they have listed countries which will do as I outlined. They will accept their citizens. There will need to be acceptable evidence of citizenship. "Right, prisoner 123456, where are you from?" "Canada." "Right, off you pop." Really?