This essay is obviously backed by a lot of thought, but this specific conclusion is a bit glib. Talking of things which are exponential, 2 trillion dollars is a few magnitudes bigger than 2 million dollars. That is more than the annual GDP of most countries.
The banks havn't lost that money under the couch cushions, they will be using it to their advantage. If it is to their best advantage to have it in excess reserves, that probably means something complicated is happening that is letting them make out like bandits.
If they aren't using it to influence the wider economy, we really need to ask what exactly they are doing with it. It is an extraordinary claim that it "doesn't seem to have gone anywhere", implying that somehow it isn't doing something.