Why can’t the Sera NX040 module configured as a responder in a two-way ranging configuration use a coin cell battery?

One of the NX040 UWB module power supply options is battery supply – limited peak current capability mode. This mode is typically reserved for tags who are considered Initiators. In a two-way ranging session, devices configured in a responder role are continuously listening in receive mode for an initiator. The current draw in receive mode is too high for a battery to support. 

Similarly, in a 3D positioning setup, the device used as an anchor (responder) will need to be externally powered.

Tag (initiator) devices can run on a battery given they do not need to be listening for other devices.