Japanese butter actually comes in many sizes, and unlike what this article says, it comes both in slabs and sticks.
You can get a 100g stick, which has pre-cut slots every 10g. This one is easy enough to break evenly with a normal knife:
https://livedoor.blogimg.jp/ddmp-mmpj/imgs/5/4/549fd611.jpg
This is not very far from the standard American 4oz stick, as far as I understand, and for most Japanese households it might be enough.
Beyond that size you can commonly find 150g and 200g slabs in supermarkets, and huge 450g slabs in bulk-oriented supermarkets (like Hanamasa or Gyomu) and specialty shops (like Tomiz). I tend to use butter quite regularly for cooking so I'm generally buying the 450g ones, as it is quite expensive here.
I guess the reason many people here mention they've never seen these kind of tools in Japan, is that you wouldn't find them in a normal house. Relatively few people bake at home here (or even OWN an oven) and butter is mostly used for toasts. If you have an insatiable kodawari (pedantry) for nicely cut butter cubes, I guess you could just go ahead and buy the pre-cut sticks.
I am using the wire grill cutter myself though, so I know it exists. And I'm pretty sure that these different cutters are heavily used in cafe's restaurants, which often serve butter toasts. They tend to serve fancy-looking toasts like this one:
https://storage.googleapis.com/mbs-recipe-and-restaurant-ass...
Yes, the toast bread is quite thick (though usually not as much as this), and the butter is frequently served as a perfectly slice on top. So perhaps having tools for perfectly slicing butter in right angle out of bulk-size slabs is not so surprising.