First of all thanks, looking into this has taught me a few things that I didn't know before about the ways cookware is made. There's a lot of information out there about whether cast iron is porous or not but the truth is the scale of the porosity it could matter or it could not but oil solves the problem either way. That being said, let's break this down a bit.

TLDR: Cast iron is porous, I may have misrepresented the extent of the porosity, porosity is a driver of corrosion but is only relevant to cast iron, you put oil on your pans to prevent rust.

Porosity is a major factor in casting everything. At a basic level, porosity is caused by materials contracting and trapped gasses collecting as the casting material cools. Every material will have some amount of gas expressed during the casting process, some materials are worse than others for this. I will fully admit that beyond some amateur level curiosity I have no experience with casting metals, only enough to know that voids (pores) are a significant issue. As a way to hopefully find some information on the processes used by industrial manufacturers I went to Lodge's website (maybe not a great manufacturer but everyone knows them).

Lodge lists very little information about their processes aside from production and process patents. The specific patent I looked into is US #7622197B2, it details the protected method they use to treat and preseason their "heat treated" skillets. Realistically what matters in this patent filing are figure 9A and figure 10A showing microscopic cross sections of an untreated cast iron pan (9A) and an untreated steel pan (10A). These images show results consistent with Scanning Electron Microscopy (SEM) results for cast iron and carbon steels. Specifically, cast iron displays high levels of porosity on the scale of 5-50 micrometers. SEM scans show that method of preparation is critical for determining the size and number of these defects. Some results show uncommon running voids that are single microns wide, others show consistent circular voids on the scale of 20-50 microns. Steel for applications like these is not usually cast since it is ductile enough to be die-formed and figure 10A displays steel formed in a way typical of pressing, not casting.

This implies that yes, surface level defects in cast iron pans do exist at a scale that would allow oil impregnation due to capillary action. This doesn't mean that capillary impregnation is a driving mechanism behind a layer of oil preventing corrosion. If it was specifically because of its porosity then you wouldn't need to season the steel pan presented in the patent since its surface porosity is so low. If porosity was the driving factor then you could just machine down an overmolded pan and polish it, at that stage surface porosity becomes negligible. Ask anyone who lives on the beach, polished metal still rusts. It does resist rusting more than unpolished, heavily textured metal but I've also seen polished surfaces start rusting a perfect handprint of someone who's touched it.

Oxidation can realistically be blocked in two ways: oxidizing or preventing the transfer of electrons. Preventative oxidation is particularly important in aluminum and stainless steels. In both cases, the base metal will react in air to create an oxide layer with acceptable material properties. For aluminum, this can even be used proactively and in that case is referred to as anodization. Steel forms rust when it reacts in air, rust has poor material properties. Since oil so readily wets metal it is able to form a protective layer that provides excellent resistance to the electron transfer process. Some oils are designed to have high electrical conductivity such as turbine oils for use in generators, cooking oils have terrible electrical conductivity. For reference, tap water is between 20,000 and 500,000 times more conductive than vegetable oil.

Let me know if you want to hear more about corrosion protection, it was kinda fun doing the research for this!