If you want to sell all your tech stocks because you think that it's irrational then take your neutral position. You won't profit from stonks going up but you won't get anything from them going down either. You've isolated yourself from them.
What the quote is advising against isn't neutral positions or pulling out early during an upturn. It's about trying to time downturns.
If you want to profit from a stock going down, you need to hold inverses like shorts or selling call options / buying puts. These inverses are always short term positions, there is no such thing as a cheap long term asset that profits when stocks go down.
Basically if you want to profit from a predicted downturns, becuase you think some asset is irrationally overvalued, then you don't just need to be right, you need to be right and time it. Because it doesn't take long before you go bankrupt holding these sorts of inverses. Aka market stays irrational longer than you can solvently hold these risky positions.
The 17 year recovery time literally has nothing to do with this btw. It's all about short term.
All this to say... shorting 1-2 years early doesn't work. You don't have the patience or capital to actually maintain a short position for two years while the market goes from 1750 to 4800. You can cheaply sit out in cash, if you want, but that's not a short position. And the S&P500 hasn't seen the kind of 300% run-up over an 18 month period that Nasdaq did in the dotcom boom.