The tweet (and me) aren't saying creating a company is "completely probabilistic", just that they're risky. Risky meaning there is an outsized chance vs. an established company that the new one doesn't exist in 5 years and the established one does (where "chance" means looking at outcomes in retrospect, not a roll of the dice).

The tweet is saying that founders becoming massively rich is a natural outcome of probability distribution -- huge payoffs require either huge bets at a smaller win multiplier (less "risky") or comparatively smaller bets at a huge win multiplier (more "risky").