Imu knows Niddhog and Nika, he fought them before. These two imo are still not in their prime. And Imu going there confidently saying he will go for a short trip means to me that he fully believes he can deal with them at the moment.
I was always a believer of Luffy needing to unlock more of his power and that he will be a full on reality warper by the end of the series
@Dragon777