I know Wusthof know better, but I suspect that they suspect that consumers don't know better. Honing does make a blade "sharper", after all... if someone just knows that their knife is dull, they won't necessarily know that the thing they need is called a "honing steel", just that they want to make their knife sharper. Looking at Amazon search results, the same decision has been made pretty widely.
If you want to make very precise distinctions, "sharpening" is not a good word, because both honing and grinding make a blade sharper. ("Grinding" is not often used to describe what you do to a chef's knife with a finely-grained stone, but that's the most precise description.)
Oh, and just to make things even more confusing: The term "honing" is actually a little inaccurate too. "Honing" in the context of machining actually refers to a form of grinding. A honing/sharpening/whatever steel mostly works by burnishing and bending the edge back into shape.
Several companies sell two-sided whetstones or waterstones are 1000/3000 grit or 1000/6000 grit (this King waterstone is the one I have), or similar medium/high grit combos. You want one of these two-siders; it's a nice compact way to get the sharpening you really need. These are what I recommend for a home use stone.
However, if your grandparents aren't used to sharpening knives at all, please reconsider giving them a whetstone/waterstone. Sharpening properly using a stone takes a bit of technique and self-education (I took a 2-hour lesson), and someone who isn't into knives is liable to either damage their blades, injure themselves, or both. Some kind of sharpener that guides the blade would probably be better for that kind of a cook.
Best Answer
Maybe.
Should you have a honing steel? Yes. Should you use it regularly? Yes.
Are you actually going to use it? ... well, many people don't get in the habit of it. And unless you're in the habit of using it regularly, it's not going to do you any good. And well, if they're like my grandparents (before they passed), it can be a challenge to develop new habits.
Once the blade has gotten bad enough to really notice, you need to actually sharpen your knife ... but that slowly grinds down the knife, and if you're not careful, you can change the curve of the blade.
I wouldn't recommend whetstones unless you're maintaining a lot of knives. For one knife, just go out and get it professionally sharpened. A professional knife store will do a phenomenal job, but for most german knives, you can just take it to most kitchen stores these days and they'll just run it through an electric sharpener (the same as you might) for around $10.
But honestly, a year is rather quick for a home chef to dull a knife to the point that you're talking about. If you're going to splurge on something, make sure that they have a good cutting board. (plastic or wood, not glass, stone, or other ceramic.