I think it's clear he meant that he is a hero to/for Americans, without anything further being implied in the positive or negative.

There's absolutely nothing unique about inserting the nationality of a hero that relates to Americans. I see it done regularly by citizens of other countries. In my observation, national pride is nearly universal.

I'm not American but I was clapping and cheering him loudly by the end of the interview. He is a hero for me.

> In my observation, national pride is nearly universal.

In my observation, it might be universal, but its degree varies greatly, with americans on the high end of the scale.

Nobody would say "a Greek hero".

In the spirit of inquiry, if the country is Greece, how likely are the following descriptions?

"A hero of Greece"

"A national hero"

"A hero to the fatherland"

"A hero to the people"

You'd say "a national hero" (the rest are not commonly used). The person can be any nationality, but he's a hero to the nation.