"he would have changed his tune so quickly after being famously against HFT"
He was still against HFT, that was part of his point. He was saying that even some of the surge of 1995-2005 might have been an illusion, because it was "only" HFT. If you feel that HFT really does raise the USA standard of living, then of course you are free to view the surge of 1995-2005 as entirely real.
https://www.nytimes.com/2009/08/03/opinion/03krugman.html
> It’s hard to imagine a better illustration than high-frequency trading. The stock market is supposed to allocate capital to its most productive uses, for example by helping companies with good ideas raise money. But it’s hard to see how traders who place their orders one-thirtieth of a second faster than anyone else do anything to improve that social function.
> What about Mr. Hall? The Times report suggests that he makes money mainly by outsmarting other investors, rather than by directing resources to where they’re needed. Again, it’s hard to see the social value of what he does.
> And there’s a good case that such activities are actually harmful. For example, high-frequency trading probably degrades the stock market’s function, because it’s a kind of tax on investors who lack access to those superfast computers which means that the money Goldman spends on those computers has a negative effect on national wealth.