I don't think Americans want to help the world any more than Canadians, Europeans, or the Chinese want to help the world. I don't see anything particularly wrong with that. It's only natural people look out for their own self interests.
An ad-hominiem attack would have been to accuse the grandparent of being a shill or an idiot, and I did neither of those. I said their post betrayed a lack of understanding (A.K.A. ignorance); I argued their points on their terms, and did not accuse them of malice or straw-man their argument.
On the other hand, the grandparent did make a statement which could be considered ad hominiem:
>"The US has always been a land for the rich to oppress the poor: the system encourages this. Those who care about the less fortunate are the minority."
Side note on HN etiquette, to say the parent comment displays a "total lack of understanding of the USA" is an ad-hominem overstatement, ratchet back the language and turn up the civility.