If it’s hidden, how does Oxfam know about it?

Unless political lobbying becomes significantly harder (globally), I don't think the growing gap between the wealthy and poor will change anytime soon.

Problem is, to tax the ultra wealthy more, it needs to be coordinated globally. Otherwise, the ultra wealthy just move to where the tax system is more attractive. Not sure if this will ever happen to be honest.

However, awareness of the problem is still important

All statistics of this type are based on substracting debt. If a headline like "Elon Musk is -100 times as wealthy as Jérôme Kerviel"[1] would strike you as absurd, these headlines are no less so.

[1]: https://www.theatlantic.com/business/archive/2012/11/meet-th...