That is not completely true, if the Zigbee Coordinator is placed very close to an Wi-Fi access point then the EMF (Electromagnetic Fields) from it can jam all frequencies even though it is primarily transmitting on different frequency then the chosen Zigbee channel, (same goes with other sources of EMF, which can be any electric appliances, cables, or power-supplies), again do try to read and follow all the tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage