eNB and gNB is the terminology for base-stations in 4G and 5G, respectively. NB-IoT is a 4G-based technology and was developed in 4(½)G – it does however fulfill the requirements for the 5G mMTC (massive machine type communications) scenario, which is why 3GPP decided to use NB-IoT (and eMTC) as technologies for 5G. So NB-IoT is BOTH 4G and 5G, which is why you will often find both eNB and gNB used in the context of NTN IoT. NB-IoT does interoperate with the 4G backhaul – Evolved Packet Core – from that perspective eNB should be the correct terminology.