No, that's intentional. It's a "reform" thingy that uses some kind of arcane building-based modifier to the "siege" weapon type to control when those units become available (there should be a few of those out East too, IIRC). Since there duly is no way to make those units benefit from the blacksmiths, they've just had the +1 bonus already factored into their stats on the assumption that by the time the reform comes up their recruitement areas will have blacksmiths already anyway. And, well, because that was the only way to not lose them an attack point in the long run.