As ricardas said, external loopback not used.
But the problem remains.
I’ve created my own calibration procedure, which almost eliminates DC using any loopback.
I’ve able to get DC down to -110 dBm (as measured by spectrum analyzer), while manual adjustments give another -10 dB decrease (uo tp -120 dBm). For now, LimeSDR can’t even measure such low signal to go down from -110 to -120 automatically (some filtering required, I’ll do it later). So, in future, I think, I’ll be able to suppress DC to almost undetectable levels.
Haven’t tested and implemented gain and phase calibrations yet.
The point is, stock calibration doesn’t calibrate properly, even if it’s technically possible. Even using internal loopback.
Another point is that, stock calibration always use internal loopback, while technically nothing prevents it from switching to external on-board.