Well, that's just weird then.
Perhaps their custom is different for Sony, or perhaps they use III for mirrorless, and I was wrong to figure III would be for EF-M only. (The one Canon-compatible lens numbered III is for the EF-M [the 18-200mm].) If that's the case, and they ever start producing RF lenses, I wonder what they'll do to distinguish them from EF-M lenses.