It’s not assuming he is making random picks. It’s saying that the stock market is a random walk, which is just a fact. He is making picks based on a system, but if that system was actually proven to find underpriced stocks, everyone/enough people would also follow the same strategy and build that value into the price, removing whatever advantage he once had.
If he’s successful because he’s a “value” investor, why aren’t all of the other “value” investors just as famous and successful? It’s because the difference between two people doing the same thing, is luck.
I appreciate that a state is working on this now even though he’s not the nominee yet. It’s an open legal question and it needs to be determined to truly give Republican voters the chance to decide who their nominee should be. If he’s going to be disqualified it should be now.