Yes. The Dexcom transmitter communicates directly with your compatible mobile device, even when it is paired with the t:slim X2 pump.
The Dexcom transmitter can be paired with one medical device and one non-medical device at the same time. (e.g. A transmitter can connect to your t:slim X2 Pump and a mobile device running the Dexcom app, or to a Dexcom receiver and a mobile device running the Dexcom app.)