There's nothing in the article to suggest that this is due to virtue though. It may be just that USV has the luxury of raising from only top tier LPs given their relatively small fund size ($1bn across 6 funds).
Before the global growth fund, Sequoia claimed that all of their LPs were not-for-profits (although excluding pensions funds is usually for transparency reasons).
Having said that, I don’t think that every top quartile fund could make the same claim even if USV, Sequioa, and Benchmark could...