What is the ruling on one who says that the fundamental nature of Islamic societies today is disbelief (kufr), and that people therein are originally disbelievers (kuffar asliyyun) except for those who disbelieve in the Taghut and adopt their (the speaker's) view, and they use historical events to prove this?

It is not permissible to declare a Muslim an apostate (kafir) unless certain conditions are met and impediments are removed. Declaring a group of Muslims apostates is an even graver error. The default state of people is sound nature (fitra), not disbelief. Disbelief in Taghut (false deities/tyrants) is a requirement of Tawhid (monotheism), but this does not mean one should withhold acknowledging the Islam of Muslims until their disbelief in Taghut becomes evident. People are judged by their outward actions, and proof is established against anyone who manifests an invalidator of Islam. The prophets called their people to disbelieve in Taghut because their people were disbelievers. Their argument that Khalid ibn al-Walid did not acknowledge the Islam of Maj'a is incorrect. Comparing Muslims to the Jews who disbelieved in the Messenger of Allah and his law is a sign of ignorance. What these young people say is manifestly false and one must be cautious of them and warn against them.