Didn't they list Kylie Jenner, heir to and made famous by the Kardashians as 'self-made'?

This is not an authoritative list, and you can imagine that people with vast amounts of inherited wealth prefer to keep that non-public. It's easier to track wealthy people who created their own fortune, as it usually involves participating in the economy in some obvious way.

It is in fact an authoritative list. It's extraordinarily difficult to hide $3-$5 billion in personal wealth (enough to land firmly on the Forbes 400) in the US.

The companies - both public and private - that can contain or produce that level of wealth are very well known and almost impossible to hide. It's rare for Forbes or Bloomberg to find new stray billionaires that had been hiding for a long time. It does happen [1], it's just quite rare, especially versus the size of the known list. You don't magically quietly generate or inherit billions in wealth, you move things when you do that and it makes headlines (either over time or when a liquidity event occurs), it garners a lot of attention.

[1] https://www.forbes.com/sites/maddieberg/2019/02/19/the-great...

If this were true, then why is there so much genuine uncertainty over the net worth of a certain Donald J. Trump?

Am I missing something? There's nothing other than the top 10.

the money-quote from this article as it pertains to this discussion is: "In 1984, less than half the people on The Forbes 400 were self-made; in 2018, 67% of the 400 created their own fortunes."

here is the full list of the forbes 400: https://www.forbes.com/forbes-400/