I think this point can be extrapolated to all people on the planet, not only to Americans))
I don't think this is meant to say that Americans are uniquely great or diverse; it's meant as a response to the racist and xenophobic turn the US government has taken since Donald Trump took office (the recent ban of immigration from seven Muslim-majority countries, steps toward building a border wall between the US and Mexico, etc).
You are totally correct. What it really comes down to is a celebration of humanity. It's just a reminder to Americans in particular.
"Our true nationality is mankind." - H.G. Wells
♥︎ this; made me tear up.