Oops... I got it. The sword was found by a non-archaeologist (one of the authors - R. Ramesh) at the bottom of some "pit" without any research of this pit itself or the accompanying material (we were only told about "an urn enclosed with a capstone"). But since the megalithic burials in this area "dated to the sixth century BCE or earlier" so this pit was dated to 6th century BCE
too. How graceful.
The article does not meet the standards of scientific archaeological research, although one of the authors (K. Rajan) is a respected archaeologist. Just pit, just sword, just an archaeologist-enthusiast, but as result we have the oldest sword made of wootz.
Just very sad. Hope I am wrong.