What does this mean? Does this mean..."someone else was very nationalistic and no other person could have been more so?"
No person could have been more nationalistic.
The reason I asked is that I was a bit confused with this.
I couldn't have been better.
This is only about me. So there is no presumed person here like the example above.
What do you think? Is my interpretation correct?
In the first example,
No person could have been more nationalistic.
there is an implied subject of comparison, probably a particular person who has already been established as the subject of discussion. A more complete statement would be,
No person could have been more nationalistic than Joe.
The literal meaning is, "it is not possible for a person to be more nationalistic [than Joe]", or "Joe is as nationalistic as it is possible for a person to be."
Such phrases are, of course, often used with an element of rhetorical exaggeration or hyperbole, so it's possible the speaker doesn't actually believe the statement in its literal sense.
I couldn't have been better.
Here the implied comparison with other states of being you might have had at some moment. A more complete statement would be,
I couldn't have been better than I was [at that moment].
There's no "supposed person" here because what you are implicitly comparing is not between you and other people, but between how you were and how you could have been.
This can possibly be changed through context:
Joe was good at chess. I couldn't have been better (even if I practiced daily).
No person could have been more nationalistic
If someone had said that about you, then it simply means that this person has never met anyone more "nationalistic" than you.
“I couldn't have been better” means you are at your best or in other words, you (feel) better than ever before.
