Any company which has sold equity, whether on the private or public markets, has a "market" valuation based on the per-share price. Private companies may also get a market valuation with a 409A valuation, but those are typically kept as low as legally justifiable for stock option attractiveness.
Perhaps, but it is a market valuation for preferred stock, not for common stock, a distinction when market valuation generally refers to the latter. Ie, companies doing a round at a $1B valuation aren't implicitly worth a billion$ - the liquidity preference affects it.
I assume it is a combination of acquisition price or last VC valuation, or market cap of IPO'd companies (which I don't think there are any). It is a useful number, but calling it "market cap" seems strange to me.
The word "market" seems pretty strange here. Are there any YC companies with a "market" valuation?