There’s other units where this may be an issue …
The Elven King for example … does the “Shardblade” count as an artifact already or can you take an extra one?
In the case of the Wiltfather it is extra strange … because, if they wanted him to be a unique Character that can’t take artifacts, why didn’t they simply make him a unique character then? I.e. like the unique Frost Giant “Hrimm” for Northern Alliance … they could have just made Hrimm an upgrade to a regular Frost Giant too, but they didn’t … yet we’re supposed to treat Wiltfather, which is an upgrade, and Hrimm, which is an extra entry in the unit list, the same, like unique Heroes … which actually does contradict the rules in the rulebook that address upgrades, if i remember right … /boggle.
I’d have to agree that an official FAQ/errata to address these inconsistencies would be really appreciated, before this becomes an issue in a tournament.