A favourite type of investment that came up now and then would be a stock that had seen a sudden sharp drop in share price triggered by some announcement that sounded scary but actually made perfect sense if you understood the industry, so it panicked investors who were only looking at the numbers and left the price-to-book low or sometimes even below 1 at the time he bought. That's probably as close to a certain profit as you're ever likely to find trading stocks.
In any case, he almost always invested in "real" businesses and for the long term. He would set stops to protect himself, but he wasn't really a day-trader in the sense we're talking about here.
The last I heard, his average annual returns were something like 15-20% before taxes and fees, which is a pretty impressive record given he'd maintained it for many years and through both bull and bear markets. I doubt most people could do what he did -- the amount of patience and discipline he had were extraordinary -- but it was fascinating discussing it with him, and strangely satisfying to watch him do well by going with common sense and considered choices against a market that was temporarily confused about something.
"Day Trading" is actively taking positions with the intent to sell in the "short-term".
If a world class mathematician says he will do something, I see the probability of success wildly higher.
It's like when you go to a writing class, and someone inevitably is admonished for doing run on sentences, and then they say "well Charles Dickens did it". Why yes, he did. But you probably aren't going to have Charles Dickens levels of talent.