Insulated barbed fire on t posts isn't common, if you run a hot wire, it is a single strand of a solid wire offset by insulators from the posts.
T posts also weren't common when people started doing this, either... in the 1880s and 1890s.
Insulated barbed wire wasn't a thing back then, either. As noted in the article, when people started doing this with barbed wire, they started ADDING insulators between the wire and the post to improve performance.
In other words, they modified what existed at the time.
Had steel posts been common at that time, I have no doubt that they would have modified those, as well.