These devices (CSS/Stingray) are going to see both your SIM IMSI number, and your device IMEI number. AFAIK, the MAC randomization that most modern phones do, is with your WiFi modem so that WiFi routers can't track you.
If you put your SIM card into a new phone, and then login to your cell service provider portal with a computer or other device, you will observe that they know the model of phone you are using. They get this info from the IMEI when your phone talks to the tower. Since the CSS is a rogue tower, they get the same info.