The network usage report shows the accumulated value of all devices. Think of 'bits in' as bits sent to the device (controller / AP). Then 'bits out' would be bits sent from the device (possibly to client or other destination).
In other words:
From a device perspective:
bits in = client requests, beacon responses
bits out = responses to clients, beacon probes
From a client perspective:
bits in = upload / requests
bits out = downloads
This is oversimplified though since there's other traffic going on in the network, but client traffic is nearly 90% of all traffic. So you're correct that this report is representing it from the controller's perspective.