Actually, it doesn't. The efficient market hypothesis gets us to some price agreement and some notion that said price reflects "available" information. (Insert reference to Paul Newman's phone number problem.)

Absent that, it's still true that company X is worth Y to person Z if Y is willing to buy Z at valuation Y. The fact that someone would do the transaction at a different valuation doesn't change that fact.

"worth" is actually complicated and it's somewhat silly to think that it can be collapsed into a single number.