Maybe what you read had some qualifier attached to it like across all VC funds "on average". With funds, the outliers are everything like with successful startups. Deal flow varies greatly by VC. A similar comment, and one that holds up, is that most startup accelerators have 0 "successful" exits.
Brad Feld's book Venture Deals [1] is a great reference if you want to learn a bit more from multiple perspectives.
[1]: https://www.amazon.com/Venture-Deals-Smarter-Lawyer-Capitali...
Many High Net Worth Individuals set up investing trusts overseas to mask their real income and these funds are usually run by investment professionals who "place" these funds.
So, yes it is tough to check the provenance of a funds LPs all the way back to the source. But if you stick to tried and tested sources - ex-founders, Pension Funds, State Funds, College Endowments, Hedge Funds, and suchlike, you're more likely than not - taking funds from a responsible source.
Of course, that is not always the case with ALL VCs.
Pranay