I have the same machine, and I wondered the same thing. My only guess is that because it does have an electric motor, and black powder is so easy to ignite, there could be a pretty serious explosion if some fines managed to come into contact with the motor brushes. I hadn't really considered that before I purchased it, but I've heeded the warning and have relegated it to smokeless loads only. It does an excellent job with those, by the way.
For blackpowder loads, I just use a modified older Dillon measure which I've mounted high over a drop tube, and it has been throwing consistent 35 grain FFF charges for my .45 Colt loads. You can see it here: starting at the 5:42 minute mark.
Until I got the Hornady for smokeless I used to use that same Dillon measure mounted on a wooden stand to which I'd attached a homemade vibrator, and it did a pretty good job for me with smokeless loads. But when I relegated it to blackpowder only, I removed the vibrator, because Hornady's warning made me think about the electric motor on my homemade vibrator causing a catastrophe.
P.S., though I linked only to the section where that powder measure is being used, your PRS Big LubeTM mold and bullets are featured elsewhere in the video.