I would be shocked if the Fed Chairman intended his comments as advice regarding the relative valuation of the price of the SP500.
In 1999 people correctly understood that this thing called the internet was going to transform life like nothing before it had ever done. And they were correct. But that has very little to do about investing in stocks. Just because you have an insight into a fantastic technology doesn't mean the price being asked to purchase stock will earn a profit. And there is no way Powell would opine on whether now is a good time to buy, sell, or hold stocks.
My reading of Powell's comments is they were directed upon the stratospheric dollar amounts being spent creating AI data centers and what that means for the economy. There is just no way he is providing advice about the price of stocks.