How many of those $10B companies are actually really
    worth that, not just an inflated result of funding
    rounds?

The parent is talking about S&P 500 companies, which means publicly traded ones. The thing where private companies have inflated valuations [1][2] doesn't apply here.

[1] https://www.benkuhn.net/terms

[2] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2955455