To me, "source" implies where the wealth came from, and it's misleading to say that the Koch brothers grew rich from a diverse portfolio of anything. They were born into oil wealth.
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)