I like that Graham quote, and you make good points. I guess when I say "investing," I mean more like - Purchasing an equity entitles you to a share of that business' income and potentially assets, but if you buy the equity on the stock market, you're not actually investing in capacity, the way you would be if you bought an IPO or even just a piece of equipment you could use to make something.
And I think people have always given the stock market more credit than it's due as being "the economy," when really the stock market is a set of signals about the economy. The economy is the people and machines and the buildings that do stuff and make things, and the economy remains regardless of what's happening in the stock market.
If a bunch of silliness happens around a certain stock, or if the stock market becomes wildly detached from reality, it doesn't have anything to do with peoples' ability to produce value.