Exactly in the same boat.
Did a thorough research of the entire market, and pixel 6 seemed like the least problematic choice - so I got it three weeks ago.
So this article is only describing the fact that JPM constructed an index that shows how tweets affect the US rates markets. The index is not tradeable afaik.
One of the markers JPM uses is likes and retweets, so I don't even think this is calculated with low latency.
There's really no reason why GDP should equal market cap. Companies are largely (especially now, since interest is so low) valued on future income, rather than current income.
Look at Amazon and Netflix, both tradin at 200x earnings. This is beacuse investors think they will earn a lot more in the future than they do now. Are they overvalued? who knows.
Authorized participants (APs) exist for all ETFs. their purpose is to make sure that the ETF mirrors the underlying assets, and to provide liquidity.
Consider a ETF that consists of a single stock. IF the stock drops, the AP will lower its Bid/offer spread, and force the ETF down to the same level as the stock. But as this is a reaction to the stock falling, there is going to be a lag between the stock falling, and the ETF falling. This doesn't mean that you can sell the ETF at a higher price, necessarily.
When you buy an ETF, you don't buy a newly issued cupon. You buy it from another market participant on an exchange - hence Exchange traded fund. What you're describing is closer to classic mutual fund. Each ETF will have "Authorized Participants" who make sure that the ETF mirrors the underlying assets.