Can a creditor obtain a judgment against a debtor who was not served papers and no longer lives in the US?
Yes. There are ways to get judgments against people even though they may not be able to be found in the US. Or put another way, the system properly needs to have methods to allow judgments against people who may otherwise make themselves impossible to find. Clearly, the person should be able to contest it if they feel they have reason, when they do make themselves available to the courts.
Just as clearly, if they cannot be found, nor can any of their assets, enforcing that judgment is difficult.