While "top quintile" might be a useful economic definition, it's not the dictionary definition, nor the one used by many Americans when discussing class (which has major social considerations, in addition to economic ones).

From Merriam-Webster: a social class occupying a position above the middle class and having the highest status in a society

Many of the HN regulars are upper income, but very few would qualify as upper class (most of us lack the political and social connections).