Sorry to bring up an older topic, but, I happened upon it while doing a search for "Yoseikan."
My instructor has talked with Hiroo Mochizuki specifically about this. Master Hiroo's response was that his father's art should never have been called aikido. It is more of a soft jujitsu. I think that's what he called it. Regardless, he said it wasn't ever pure aikido. You also have to remember that Minoru left O-Sensei before the name was changed to aikido.