A hacker is an individual or a hacker group. When a state is involved we can think more about the typical work of intelligence agencies and/or cyberwar.
Why not, maybe we can get the original meaning of the term hacker back at some point.
Threat actor is a more generic term that covers all kinds of attackers.
For example, it could be an insider who exploits existing privileges (no hacking involved). Or it could refer to a nation state or organised crime (not an individual).
and "nation-state" is the new "sophisticated sql injection"