Overnight means you take the other side: buy at the close and sell at the open. (This is a bit more complex conceptually as you would never settle the trades and that may be problematic regarding dividends and other corporate actions.)
The move to T+2 settlement last decade reduces the requirements, and T+1 later this decade would reduce them further.
Dividends and corporate actions aren't a big deal. Those all have announcement dates and record dates. If you hold the shares at the close of market on the record date, you will get the dividend or other proceeds. So if you always manage to buy at close, you will always get those benefits. The only complication would be if you want to oppose a merger and have standing to sue; or I guess if you were an injured party in any other shareholder lawsuit. Lots of tax paperwork too.
You never settle the trades? Dividends etc?
Hmm maybe I am 5...
Edit: ok yep so I guess what if they repeated the analysis leaving some time around open/close for chance of trades to settle, would the effect disappear or would this chance beef the key factor for the reported gain?
The analysis is flawed if it doesn't include this as the gains they describe seem essentially unrealisable.
So is the real issue that maybe someone has immediate settlement when the rest don't?