Full duplex refers to simultaneous two way communications.
Yes, there are 200 byte buffers on the radios but after over head, the streaming limit of the XBee 802.15.4 modules is ~80kbps.
A Streaming limit is defined as the rate in which a continuous data stream can be sent across a device or connection.
Since the radio is a Half duplex device. That is it can send or it can receive at any given moment in time, your rate in which you interface with the radio must be equal to or less than 1/2 of the streaming limit for the device to act as a full duplex connection.