What I see on our 2020 is Lunar Silver paint without the clear coat on some parts toward the front end of the engine bay and on the firewall below the plastic shield I see a greenish/grayish primer coat. I will check our 2018 Ridgeline tomorrow to see if it was treated any differently and post back.
In addition, paint and clearcoat are not particularly good at corrosion prevention on their own ... it is the actual anti-corrosion coatings on the metal prior to primer, paint, and clearcoat that matter in this regard.It is common that areas 'out of sight' (underside, engine bay, underhood) are not clearcoated (by many manufacturers, not just Honda). Our '06 is this way, and has no rust issues.
The only things that have visibly corroded over the years are the brackets that hold stuff in place.