How would you distinguish an investor billionaire who became rich by chance from one who chose wisely, after the fact? If you have access to information regarding how they made their investment decisions, you should be able to tell them apart, but in most cases that information probably isn't available.

One of Jon Ronson's books (probably Lost At Sea, but I've since returned it to the library and no longer have it for reference,) features interviews with individuals at levels of income on steps of about five times apart, and one of those individuals, whose wealth was not in the billions, but was in the hundreds of millions, had made his wealth because a college friend introduced him to one of the founders of.... some major online corporation, possibly Amazon, so that he could provide startup money which he'd received from his father's business. So his acquisition of his wealth was largely effortless and due to chance by his own admission.

I don't know to what extent this is normal for very wealthy people, but his perception seemed to be that it was quite common.