I dunno, I'm Jewish and I wouldn't mind this particular remark, and a sister comment claimed it seemed typically Chinese just as much and claimed to be Chinese themselves.
One point here is that it's seemingly OK if it's not derogatory plus one is talking about one's own group. Another point is there's a continuum between nationality and culture, and it ought to be OK to point out that something is specific to a culture, and American specifically is very likely to be used to label a culture and not the people as in a bunch of guys and gals with specific genes. This is not to say that one ought to agree with grandparent's views on American culture, just that those views probably aren't racist.
BTW I personally try not to say this sort of thing since people might be offended all too easily, so I guess I agree with you in that way.
This remark would be considered highly offensive and racist if it were uttered about most nationalities.
Would you say that to appraise the sentiments of, for example, a Jew? Of course not. Everyone would rightly condemn you for it.