But keep in mind that they have more than doubled their net worth in the past 8 years. And in those 8 years it was a diversified portfolio that more than doubled their wealth, so given that more than half their net worth was accumulated with a diversified company I can see where Forbes is coming from.
http://www.huffingtonpost.com/entry/koch-brothers-net-worth_...
For what its worth Bloomberg also lists their industry as diversified.
TL/DR you and Forbes both have a point and the only reasonable thing is to is to take Obi Wan Kenobi's advice and agree that you're both right "from a certain point of view"
Although I imagine such returns, with acceptable risk levels, become harder to find when you are dealing with 11-figures.
$12.7B from Georgia-Pacific (a "pulp and paper company")
$10.6B from "Other Businesses"
$9.6B from Koch Pipeline (petroleum)
$4.5B from Flint Hills Resources (refining/chemicals/biofuels)
$3.8B from Molex ("connectors and interconnect components")
$3.5B from Koch Fertilizer
$3.0B from Invista ("polymers and fibers")
all other sources below $2B
Looks pretty diversified to me. Just because they made their fortune in petroleum doesn't mean that's currently the largest source of their wealth.
Source: the Net Worth Analysis from the RICH function on a Bloomberg Terminal
I guess another way to put it is that a very human mistake to make is to infer something and assume it was implied, then use that assumption to form a dismissal.
I get that your particular interpretation is likely meant as some subtle manner of dismissal of the Koch brothers (something along the lines of how they didn't earn their wealth would be my guess) and I'd like to encourage you to state such feelings directly. It's far more interesting than low-grade emotional rhetoric that just turns into boring no-light-low-heat discussion.
When looking at a list of billionaires, it's natural to wonder "where did this fortune come from?", and in the case of the Koch brothers, the answer is "oil". I don't think it requires a deep dive into my psyche to defend this interpretation of the "source" field, though I'm always happy to swim around down there.
What's the source of my wealth in this case?
(this seems to be a clarifying exaggeration of what is being asked here)
Congressional policy decisions always have to keep future fundraising prospects in mind. It is not a stretch to describe candidates who depend on Koch largesse to win as "bought and paid for." Keystone XL (a certain pipeline Koch Industries stands to benefit disproportionately from) is a controversial issue, and it ought to be considered with an eye towards policy impact rather than next year's electability.
[1] http://www.nytimes.com/2015/01/27/us/politics/kochs-plan-to-...
http://www.dailykos.com/story/2014/4/1/1288957/-Sign-the-ple...