First of all, it's talking about the metallurgy of the 16th century and after, which is after Japan had imported blast furnaces. Secondly, it ignores the amount of labour needed to actually produce refined steel from iron sands, which ultimately dictates the quality of the finished product.

This isnt a debatable topic, any steel made from iron sands before modern electromagnetic sorting contains a large amount of impurities when compared to steel made from rock ore.

Even during WW2 the Japanese had a hard time producing high quality steel even with the use of blast furnaces, because the iron sands contains a large amount of titanium.

This blog which falls over itself trying to engage in revisionist history, can only claim that the quality was "perfectly fine"....not good.