Yup. That's correct. BMW planned to release assist with integrated Bluetooth but for whatever reason it didn't make it and the ecu only is able to do assist. No Bluetooth despite the logo and BT pass key.

I had a m3 just like yours from 2003 with exactly the same assist ecu. Because I wanted Bluetooth I ended up doing a BT retrofit even before BMW offered it.

Anyway, simply purchase a newer ecu with Bluetooth. Bit of programming at the dealer and that's it. All the hardware is there. You have the microphone and Bluetooth antenna !!