so joy boy was the previous nika nika df user , and for 800 years no one ate this df .....
which makes me think that teach who's knowledgeable about all devil fruits wasn't able to find this particular df so he was looking for the df that counters the other dfs
but teach didn't care about luffy's df in imple down , so he didn't know that gomu gomu was actually the nika nika df he was looking for