Statistics.
They aren’t really “need blind”.
They are “need blind” on an individual level, but not a group level.
They set admissions standards in such away as to favor students from more well-to-do backgrounds. Standardized test scores are one of the best proxies for family income and socio-economic status-- so if a school is looking at test scores, it is looking at family income as well. Again, not on an individual basis – there will be rich students with poor test scores, and poor students with high test scores – but on average, a student body with high SATs is going to have more wealthy students than a student body with more moderate average scores.
Colleges also use binding ED lock in full-pay or nearly full-pay students early on. Most end up taking 40% or more of their classes that way.
And the holistic admission process also provides other insights into family income. The college admissions process values activities and accomplishment that generally cost money, and are hard for people on the lower end of the economic spectrum to access, whether it is years of music lessons or participation in EC’s and summer programs with accompanying participation fees and travel costs.
It works, year after year, because even though individual students may be outliers, the ad coms actually have a very good sense of which students will need substantial financial aid and which won’t.