Investor is too kind a term. They did not exactly buy those papers at nominal value.
They didn't agree to the restructuring amount, and kept the original bonds until selling them to the current bondholder; and as far as I understand, Argentina has not kept up the payments to those who refused the restructuring, that's the whole start of those court proceedings.
They are holdouts precisely because they didn't agree to a haircut. Argentina declined to write a collective action clause into their bonds so that they could sell them for a higher premium. This is the price associated with accepting that risk.