Think about if you were a writer. You can't say something's old if no one reading HAS EVER SEEN THE TECHNIQUE BEFORE. Then everyone would be asking "where did that move come from? - I've never seen it!" when that information hasn't been released at the time. So either kishi hadn't thought that far ahead (to where Kakashi actually gained the MS in the past), or he did the right thing and called it NEW because it WAS new to the readers.
no he thought of it but this is what happened
kakashi got hit with the ms and b4 he could even know what happened after his long fight with the mist anbu he passed out and with no one around he didnt know what was up he just past out
when he woke up he had the same sharingan he had since obito gave it to him and would have simply thought....arhgghg what happened ( ive been knocked out b4 in my life i know what its like)
so he wouldnt have known what happened and then years later we dont know what happened so i wont claim to know like some ppl but years later he had it re activate over something and when he felt the strain he would have been like whats up and figured it out
remember kakashi is a genius
if kakashi knew he had the ms and he knew he didnt have the stamina for it as a genius he would have done what he did later one and that is train his body to handle it and we know a genius like kakashi wouldnt need 16 years to figure out whats up if he knew