However, their argument is incorrect. Extension of the statute of limitations has nothing to the principle of non-retroactivity of criminal law. Article 3 only calls for extension of time for prosecution, it does not make non-crime then a crime now. Plenty of civilized nations accept extensions of statute of limitations. It is the limitations period that is in force at the time a person is charged and prosecuted, not when a person commits a crime that is relevant.
Nothing here makes a new crime or charge retroactive.
Case in point, under the German Penal Code 1871, crimes such as crimes against humanity, war crimes were punished under crimes such as murder, manslaughter and unlawful deprivation of liberty. The limitations period for these crimes was between 10 and 20. Because the constitutional prohibition of ex post facto criminal laws, the newly inserted genocide Article cannot have any retroactive effect. Because of the change in the limitations period, Nazi war criminals and those guilty of murder or manslaughter during the Nazi regime could be lawfully charged. Another point in relation to your article is that while sympathy for the victims is an important virtue, often virtue has nothing to do with justice.