Most common finance myth: there's one "true" gold standard valuation method. People who are negotiating against you will like to spread this myth, and of course they say their valuation is the true one.
Finance Reality: There are nearly infinite valuation methods, none are the true one. Total future cumulative value of an asset is always, by definition, a prediction of the future, regardless how much bullshitting VCs may tell you that "they're not trying to predict the future."
Nuance: It is fair to say that market value, in retrospect, was the true value at a particular past time. How far into the past you have to go to escape all associated legal risks of making a trade and all possible clawback situations, is an open question that complicates this though. Further, these days it's getting harder to say which of the many parallel trading markets was the real one, even for past market value. There are near infinite ways to coalesce multiple parallel past market price histories into one price history, as well.