Yes I think they are made by Juin. Yokozuna provide a full kit (calipers, rotors, compression-less housing and nicer graphics). If you have mechanical levers they are the most convenient option.
Ultimos on a rear post mount:
Listing update: just added some GRX hydro calipers!
Anyway, are yokozunas not just juin techs but with better cables? You can get the cables separately from.one uk supplier (can't remember which)