It may be true that "startup" success is mostly random in the YC/modern Valley-VC culture of throwing a fistful of darts at the board and hoping that at least a few stick.
This is why when you only have one life that you should put all your efforts into what the VCs disparagingly call a "life-style" business rather than be one of their random darts. It doesn't matter to them if you end up as one of the darts on the floor, but it will matter greatly to you.
"Startup" implies venture capital, which means the founders' early understanding of the business is going to be amplified by an infusion of cash, with the expectation of rapid growth.
Business success can occur slowly, perhaps growing 5-10% every year (or maybe not even growing at all, and just remaining profitable).
I think of it like golf: bootstrapped businesses are like trying to use a putter to get the ball from the tee all the way to the hole. It's probably going to take you a long time, but if you pay attention you have a good chance of eventually succeeding. VC is like a big driver club, so you can get the ball to the hole faster but if you make a bad shot then you're screwed.
Business success, however, is not mostly random.