Indeed, it is underdetermined what the alternative source code is. Sometimes it doesn’t matter (this is the case in most decision problems), and sometimes there is a family of programs that can be assumed. But this still presents theoretical problems.
The motivation is to be a nonrealist about logical counterfactuals while being a realist about some counterfactuals.
The summary is correct.
Indeed, it is underdetermined what the alternative source code is. Sometimes it doesn’t matter (this is the case in most decision problems), and sometimes there is a family of programs that can be assumed. But this still presents theoretical problems.
The motivation is to be a nonrealist about logical counterfactuals while being a realist about some counterfactuals.