No, the word "just" in "He's not a hero. He's just a guy" indicates that he's not a hero. "Just" applies to the "just a guy" part, not to the "entered business transactions" part.

In conversational English, the phrase "He's just a guy" carries an idiomatic meaning along the lines of, "This person is no different from anyone else. He has no special power or influence or insight." And that might be true with respect to insight, but it is clearly not true with respect to power and influence. And that is why, when PG says something tone-deaf, it can hurt more than when some rando does it.

I agree in general, but not in this specific case. "He's not a hero. He's just a guy" makes it such that "he's just a guy" is applying specifically to the "he's not a hero" bit. I didn't take it as anything more than that, and I don't think the author intended it to mean they believe Graham is just an average person with an average amount of money, power, or influence.