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I think what's left unsaid in the paper is that if a Lorentz-invariant speed exists, then it is unique. (This is easy enough to show.)

Therefore, once you show that the speed of light in a vacuum is Lorentz-invariant, then it has to be the unique Lorentz-invariant speed (i.e., the universal speed limit).

Yes, this strikes me as an accurate framing of the situation's epistemology.:)

> That's all fine, but from the moment Maxwell's eq's are invoked, we enter an empirical framing, not a first principles one.

Yes, of course. First principles only leads you to the conclusion that there is a reference speed. It tells you nothing about its actual value. In fact, first principles cannot rule out the possibility that the reference speed is infinite. They can only lead you to suspect that it might not be.

Yes, this one also strikes me as an accurate framing of the situation's epistemology, Ty for clarifying :)

> Maxwell's results follow from observations of the behavior of charged phenomena, current, magnets, & electromagnets

I mean, historically yes. But you can define the fields as purely postulates if you like. The theory actually requires very little input to derive.