First off, the product you are referring to is a Mesh based product and will form a mesh on its own.
Next, the Znet 2.5 code you are referencing is about 5 years out of date and not supported. I would strongly suggest updating to the current 2xA7 Zigbee based code.
As for API mode. API mode is not required. API mode is only an extension of the radios firmware which uses Frame based communications for the data flow in and out of the UART of that specific module. Both AT and API versions will perform a mesh network and will use the lowest path cost to send the data from Point A to point D.