The indicators I mentioned are associated with a nation state actor because the attacks in which they were identified were attributed to a nation state. In other words,the selective targeting and sophistication match the interests of a nation state.
Geo-political intelligence analysts and sometimes field officers (aka spies) also play a role. For example,if a specific journalist was attacked while working on an article critical of a specific country and at least one indicator was traced to an a real human associated with that country's spy agency the a confident attribution can be made.
The term "nation state" does sound a bit fancy but it is different from "country" in that many nation states do not act on behalf of their people or to protect the interests of their conuntry. If you use "country" that would include the people of the country while "nation state" simply means the organization running the country. Plus you have situations like taiwan,hong kong and chechnya where those nations are effectively their own nation state but they are under the control of a different country.
Here is a good reference attribution of APT3 (Chinese Ministry of State Security (MSS)): https://threatpost.com/apt3-linked-to-chinese-ministry-of-st...