There have been plenty of these retroactive reviews of the 1990s tech boom that have questioned whether it was a bubble, by changing the definitions that everyone else uses to define valuation ratios and bubbles. I've heard it argued that the dotcom bubble wasn't a bubble, because the valuations were justified in the moment due to future expectations, until they weren't, which just sounds like the definition many others use to define a bubble.

As to Cochrane, even if the value of Amazon is argued to have justified the tech sector valuation, the fact was that the tech sector valuation was not all, or even mostly, concentrated in amazon, which is why it was a bubble.