However the Imperium still uses gunpowder for their weapon, yet this isn't even 21st century technology. They also still use radio, grenade launcher, rockets, missiles, spotlights, and commanders/commissars got a sword. I guess the Horus Heresy stuff is from banned times of doom they kinda want to forget, but if my memories are correct the LandRaiders, Baneblades and Titans are from that techologic age (and considered as relic for that reason).