It doesn't do much that is useful in the uncle's situation.
If you register for it then you won't get unsolicited credit offers from companies you don't have an affiliation with. So American Express can only go pulling random people they aren't affiliated with a score of over 700 or whatever and send card offers only to people who haven't opted out.
However, it doesn't prevent companies from marketing credit offers to people they do have an affiliation with. So if you have an AMEX card then American Express can market other American Express cards to you even if you signed up for prescreen opt out, and your bank can market their cards to you, and your airline rewards program can market their cards to you, etc.
If you register for it then you won't get unsolicited credit offers from companies you don't have an affiliation with. So American Express can only go pulling random people they aren't affiliated with a score of over 700 or whatever and send card offers only to people who haven't opted out.
However, it doesn't prevent companies from marketing credit offers to people they do have an affiliation with. So if you have an AMEX card then American Express can market other American Express cards to you even if you signed up for prescreen opt out, and your bank can market their cards to you, and your airline rewards program can market their cards to you, etc.