Their average trailing earnings yield equated to P/E ratio of 44 a decade ago, but they were all trading at only 4.2-5.4 times their respective average earnings per share in the last three years. (Apple too, the only difference being that it was also cheap on trailing earnings at the start). You'll generally get an above-average return buying at any multiple of (say) 8-year-forward earnings under 10x. In short, they were trading at less than half of fair price.

The second most important part of that comment is the "pretty-darned-sure" part: future earnings haven't happened yet. You'd have had to know each business pretty well to estimate with confidence the earnings trajectory that we've seen.

This is the key weakness of value investing. In short, they chicken out when there is no margin of safety, because future earnings are so unpredictable as they were for the tech companies you cited. No way you could project Netflix's earnings, for example, without considering all the bad things that could have happened. Or Amazon's without knowing AWS will be a money machine. Of course people buying for "bad reasons" were buying on hope and faith in unknown future good news, visionary leaders (Bezos eg) etc. Not based on financial statements.
Damodaran tried to capture this in numbers using "growth assets" added to their balance sheets but ultimately it's an exercise in futility to predict earnings when the landscape is ever-shifting. Dispersion in future returns overwhelms the trend line.
Obviously I don't know what the answer is, and it's a problem even Graham recognized, but the world moved at a snail's pace in his time compared to ours. Now the only stocks value investors can comfortably buy are somewhat slow growers / no growers. Fast growers are so popular that they will forever be too expensive by numbers (considering MOS) and yet with forever better returns than value stocks. Partly because of the demand (similar to crypto) and partly because they can raise money at will to develop or buy any new potential money machines.