vanguards very large and they're a mutual fund, so they have a lower profit motive than most brokers for it. they're basically the standard retirement fund company now
What I think they're referring to is their sheer size (ratio of customers or $ invested to staff) has meant historically the annual % fees on mutual funds and other investments have been lower than at other institutions.
It's... less unique to Vanguard these days, as several of the large providers have equivalent low-cost funds you can invest in; but 15 years ago it was more significant, iirc.
