This native speaker thinks you have a point. The “consistency” is a property possessed by the graphite, thus “has”. The “is” suggests that graphite is to be directly equated with consistency.
Comparatives are always tricky in English, but consider:
“Ball bearings, which have the smoothness of plate glass, ...”
You would not put “are” for “have” in the above. You can generate a lot of similar examples.
Nice examples. It would take more of a grammar nut than I am (and I read Garner's book cover to cover) to untangle it. I'm not able to detect any relevant search terms, either.
Comparatives are always tricky in English, but consider:
“Ball bearings, which have the smoothness of plate glass, ...”
You would not put “are” for “have” in the above. You can generate a lot of similar examples.