For good swords, you're correct. They were not good swords at the time of the mongol invasions. That was my point. They were shit, because they were not primary weapons whatsoever.
It was a very short set of battles, too, so it's not like they had much opportunity to learn, especially given that the Mongols fought so dishonorably (which is to say, they didn't engage in miniature duels on the battlefield as the Japanese had been accustomed to doing).
It wasn't unique to Mongolian armor. It was semi-unique to their specific type of armor. They had brigandine(layers of leather with metal plates or rings) armor, not the plates of early retainer armor. It's much thicker in terms of material, which means more grip and opportunity for it to break. You misread my narrative, or I was unclear. I was just trying to say that it was during the Mongol invasions that they got that wake-up call. There was a drastic overhaul in swordsmithing techniques following that, and the blades of what we think of as the Samurai era were of a far higher quality, at least for families who could afford them. Mind you, they weren't ridiculously amazing. High quality, yes, probably higher quality than the typical European swords if only by necessity/selection bias, since they were status symbols and there was so much less iron available to work with(only the best smiths were hired, basically, even more so than with Europe, at least for cavalry sabers(which is what katana are, I'll remind you)). But the edge was brittle, and chipped easily, requiring sharpening quite often.
Hindsight is 20/20.