Found the cause and solved. what happened was:
1. Plugged the host PC only into the STLINK USB-C. with SW1 set to 1, the device booted from EMMC and debug data was shown on USB UART. Assumed based on the MPU running, it would be enough power for the DFU mode.
Once the USB connection was shown in ST CueProgrammer but later on it disappeared and USB connection was never detected again.
2. Bit ofcourse, DFU is not provided via STLINK hardware but via the DRD , CPU OTG USB interface. Though after connecting the USB DRD into another USB host connector there was still no DFU mode.
3. After connecting the STLINK USB-C to an 5V charger and a USB-C cable from DRD to the host DFU mode came up.
This particular PC might be too old to provide enough juice. but wonder why this board does need more power than a USB-A 3.0 connector can supply (0.9A) . Perhaps some may run into the same issue.
Cant use DFU mode and ST-LINK at the same time, but the board works.
