In truth, I have the same question. In fact, steel at 65RC, in a commercial scale would not only be impossible to make and sell, the sheer number of blades that would fail would be huge.
Here is an old photo I copied here from the forum on average hardness of razor manufacturers;
Attachment 284245
And the above chart is for western style razors, kamisori were going a bit higher still. I'm baffled on how but more importantly, why would anyone do that.
As for the "it's impossible to get steel harder than 66RC", it's simply not true; there is no law similar to absolute zero or speed of light; 1% carbon, correct heat treatment, quench, temper, dip in liquid Helium to maximize martensite formation & voila! You can do it yourself, with enough money, practice and patience. As for being chippy, I'm sure you are right, that thing would be more like cermet than steel. I haven't tested such a piece although I do hope to.
I'm not saying that I have seen, or verified its existence. Only that at some point I read about it, and thought "holy mother of God, how was that done and why?"
We have some very capable blacksmiths, quite possibly some of the best of our age here in the forum who know their stuff, usually in the "forge" section. I enjoy reading about their accomplishments, you can say they are the "astronauts" who walk the moon, I'm the lowly "astrologist" watching from afar.