Around 1986, I had the headers for my Hemi car coated, basically the same deal as Jet Hot - it was a place south of L.A. that did aircraft & racecar stuff, but I don't recall who.
Those headers still look fine; showing a bit of age at the collector junction, but no peeling, flaking or rust.
About 10 years later, did the same to the racecar headers, different place, and they still look fine after some polishing on them.
Neither car was ever subjected to rain or outdoor storage, nor used as a daily driver.
Another car, which was a daily, I painted the headers with Eastwood's stainless paint: it lived outdoors in a fairly dry environment, and it took 2 years for any thing like corrosion started to show up.
So, my guess is that TTi & etc sub the coating out to the lowest bidder; it may just be powdercoating.

Back in the boat-racing days (inboards, tunnel hull, fresh water) wiping the painted headers down with ATF was part of maintenance, and seemed to work.
The WD-40 idea of course displaces moisture, but I can't recall trying it. Seems it would burn right off, and a re-application every time out sounds pretty ridiculous to me.