Sayintentions has no traffic injection feature and you can´t listen to the AI traffic comms either, only to other player´s comms or the generic canned messages. Native traffic management (as in Beyond ATC case) is a planned feature as far as I understood.
For the time being you must use the simulator native traffic or an external solution like FSLTL injector for generating traffic when using Saytintentions and currently Sayintentions will take it into account for traffic proximity warnings only. So FSLTL works as it´s just standalone in this use case.
Cheers