I have Tojiro Pro knives myself, but, have been really wanting to get one of those Onion's.
First, please be sure to treat these Japanese kitchen knives with respect! They are very fine cutting tools. I got rid of all my high dollar European kitchen knives after trying the Japanese knives. There was just no comparison for me. The Japanese knives were just a far better product, IMO.
The reason for not using an electric sharpener on it is because electric knife sharpeners are extremly coarse, high abrasion sharpeners. Not too different than if you took a Sebenza and sharpened it on an 8" bench grinder. Massive overkill. Japanese kitchen knives have extremely thin, acute edge that average around Rc60 and above. It's a given that if you use an electric sharpener on them, you will destroy the edge and quite possibly the temper too. They sharpen up quite fast on fine and ultra-fine ceramic stones. Some people use the coarser grit stones(120 grit) to re-establish a bevel then go to consecutively finer grits ending somewhere between 1000-8000 grit.
You can use the Sharpmaker, but, I'd use only the white stones though unless you need to do some edge work like a nick or something along that line. Actually, I normally use a Norton combo stone of 4000/8000 grit. Japanese water stones are what is normally used on Japanese kitchen knives.
The lowest grit I've used thus far was 1000 grit.
Japanese kitchen knives are an outstanding product that should never be put in the dishwasher, or used to cut drywall or something along that line.
My only regret about these knives was not having them in my kitchen 10 years ago.