That specification is for the module in storage (not deployed/installed in a carrier) or during transportation. Refer to the part of the note below that table: “Duration based on product being packed and stored in a controlled environment without power on.”
For the actual operating temperatures, the range is much wider:
Does it mean that when I mount the nano onto our own baseboard and then power it on to flash the system, I no longer need to worry about this storage temperature during transportation?
Correct. The storage temperature in the spec you referenced is pre-deployment, for example while it sits in storage waiting to be sold (by a distributor) or in inventory waiting to be put into service. The storage temp and humidity directly relate to the storage life spec in the same table: as long as the module stays in those storage ranges, it remains deployable for 5 years from the NVIDIA shipment date. Once the module is in service, the operating temperature specs apply.