This is an obvious and accurate criticism. But it's also worth considering that large financial firms are both the best equiped and most motivated to obscure their transaction activity, so I don't think it should be dismissed trivially.

The question is: how could one find evidence that justifies or refutes the idea that the scale of firms does in fact fit zipf and that measure is being obscured at the tail?

Also, I'd point out that the original post did not differentiate between financial and non financial companies in the way you do between points 1 and 2, so your characterization of what fields are related or "semi-related" is your own creation and not part of the original claim.