Earlier quoted context omitted.
The intended lesson is restated at the end, in case you missed it: Or we could...start valuing stocks based on fundamentals. Valuing stocks on putative future profits based on users, or on comparative values based on other inflated stocks, or based on the price someone paid for some fraction of their shares last week, is not really a solid way to try to calculate value for investors. It's a difficult problem and no-o…
Who is "we?" The market can stay irrational for longer than you can stay solvent.
Stock markets behave mostly irrationally in the short term (see Random Walk Down Wall Street), but that doesn't mean you shouldn't attempt to ignore the irrationality and get a little closer to the objective truth - in this case by valuing companies based on what they earn and could earn, not what someone else paid for them or might pay for them.
Depends what you're after I guess - if you are trying to make lots of money in a few days/weeks/years with borrowed money, you'd absolutely have to attempt to predict the irrationality of the market or go bust. On a long time scale with money which is not borrowed/required, you can afford to take a longer view based on profits.