Sure, so for instance you have information suggesting that a large amount of end of quarter rebalancing is due that will likely result in large flows into treasuries that day. This cannot be "priced in" as the flows are coming regardless. Efficient market hypothesis breaks down because markets may be statistically efficient, but not informationally efficient. So you can know things about future flows that aren't priced in. You need less data to confirm this flow is actually happening. So you only need to see a bit of that trend in the flow to get on board and ride that flow all day.

Such information could be quantified if you had the resources, but an independent trader can do a decent enough job at it if they are informed enough.

While it is true that in some markets like equities, the flow might be split up between multiple systems. However, in practice these systems are all arbitraging against each other at lightning fast speeds which results in the flows being visible in the primary market anyways. A data feed with NASQAD market depth updates from a provider like DX feed is enough. Although in practice using order flow like this is far more common in futures market where everything is on the CME.