[blargey]

They're pointing to the fact that most of a given billion+aire's net worth is ownership of a highly valued company (stock). Their billions are literally just the market-assigned value of the control itself, so it falls upon people suggesting a wealth cap to come up with an arrangement that can divest them of that value without divesting them of the ownership/control rights that said value is being derived from.

Of course, control of a very large company is itself the sort of power that wealth caps are supposed to curb, like when people buy newspapers / social media platforms / etc - so you could also just proclaim it a feature instead of a bug, but it still warrants a defense.