These companies manage ETFs, so instead of purchasing shares from 500 companies tracked in S&P 500 (and paying 500x the commission), you can just purchase X shares of the VOO ETF. On paper, it looks like they own everybody else, but in practice they are just proxies.
I think the GP's (heavy-handed) point is that the proxy ownership doesn't extend to voting rights: when you own shares in an ETF, there is no way for you to individually vote based on the underlying constituent stocks.
(This is arguably a bad thing for long term economic health, since broad-market ETFs have a dampening effect on otherwise healthy amounts of speculation. But I think it's a far cry from saying that they "control" the market, since it's a mostly passive effect.)
