The crew cab shortbox option didn't come out until 99 if I recall correctly, but even if it was 98, it had to be a k2500. Not a 1500. They sold k1500s crew cabs in other countries, most likely that has been brought across the boarder from Mexico.
The bigger issue I'd say that it's seems scamy to me. And the add is most likely a scam of sorts.
You most certainly can get a "silverado" then as the Silverado was the top tier trim level designation for years before they took the name and started calling that gmt800 line of trucks "Silverado" with trim levels being called LS,LT, and etc.
The duramax was actually designed to fit into the gmt400 chassis and body, as it had been in development for a few years before they rolled it out in 2001. But gm decided to roll it out in the new "HD" 2500/3500 trucks in 2001, instead of releasing it in the gmt400 series trucks and instead rolling with the fuel efficient, but non powerhouse 6.5 until 2001.
This was a mistake in my opinion, but since the brakes were vastly better in the gmt800 trucks, it does make some sense to keep it out of the gmt400 chassis, as the towing capacity would be a bit limited in the gmt400 anyway.
The ecoboost thing. Just proof that the seller doesn't know what it is, and it's a scam. Imo.