That’s one question and I am certain the ancients didn’t have a systematic nanotechnological approach to this art of sword making. They probably discovered a technique that unwittingly utilized a nanotech principle that they discovered would make swords stronger/sharper/not rust/etc.
They are claiming that forge welding ie Damascus steel was first invented in India. This is a process where by steel is folded onto itself when hot and then welded by forging with a hammer. The process is repeated over and over till there are many, perhaps hundreds of layers. Carbon is incorpotated into the iron in the process and very hard steel results. It is also known from very old times in Japan.
The article makes the claim but offers no hint as to hoe the process is nanotech.