Oh! I looked a bit closer this time and it looks like it is using both WDM and TDM. The backward and forward channels are centered at different frequencies while channels meant for different users are TDM. In this example though, there was no time division demultiplexer explicitly used. The BER for each separate user is calculated with all the bits, even the bits outside of their respective time window.
The upstream signal’s TDM nature can be seen if you open up the subcomponents for each user (ONU). They each have time controlled switches that only allow transmission in certain time windows.
Regards