Salt is a lubricant.
I've never heard of salt having lubricating qualities. I'm not saying it doesn't, just never heard it. How much road salt gets onto the braking surfaces, and how long does it stay there? I have no idea. I've never had to deal with salted roads.
As we all know it corrodes ferrous metals. Rust or blackened metal of a grey iron brake disc has a lower coefficient of friction than good shiny metal. That's why discs in good condition are so (and so massively under-appreciated) much more efficient at braking quickly. The MOT brake roller test is such a minimal standard for brakes and doesn't take into account the effect disc degradation has in a dynamic, high speed hard brake application. Not only is initial brake response delayed but so too is the maximum braking effect and the speed the brake reaches maximum retardation (which then needs ABS intervention) can be very much lower, or may simply not be reached at all.
But, it's my understanding that the pads are not in contact with the disc metal, but rather are in contact with a microscopically thin film of pad material that has been transferred onto the disc surface in normal use. The braking effect (friction) is between deposited pad material on the disc and undeposited pad material on the pad (though this is an over simplification because material is constantly being 'swapped' back and forth between the pad and disc surfaces). The layer of deposited material is so thin that it is more or less invisible (though can possibly be seen if there are abnormally thick deposits on the disc, usually uneven).
So, assuming this to be absolutely correct and not an over simplified understanding, it shouldn't matter what the disc surface actually is, whether it be steel or cast iron, or whether it is oxidised or not (shiny bright or darkened), as long as it forms a sound foundation for the deposited layer of pad material...
Having said this, there must be times when and conditions in which there is 'clean' metal on the disc faces, otherwise the disc would never wear and become thinner over time...???
If anyone cares to measure braking distance of corroded brake discs from highway speeds against that of bedded in new brakes, the result will evidence my point.
Unfortunately it is that same reason why many people often think the new brakes just fitted are an upgrade. Even rubbish quality new brakes will be far better than expensive old brakes which were in poor condition.
There may be other factors with the 'old' brakes vs new that might be affecting how well they work. Or, may affect pedal travel and feel and how this may influence the perception
of the performance of the system. This might include such things as glazing of the pads and / or disc, pad taper (in one or two axes), disc taper (thinner near the outer edge, typically), looseness in caliper mounting pins, pad sticking etc.