Yup, this is correct. I was wrong about this being 330 only and the same rule applied in the late model 320 I tried this in last night.
In managed DES the FMA showed THR IDLE, but the N1 was at about 42%. On selecting Open Descent, the FMA stayed at THR IDLE, but, the legend IDLE appeared at the top of the E/WD just above the N1 guages, flashed for about 10secs and became constant along with the N1 falling to about 36%, so properly at Idle. Re-selecting DES and the little bit of thrust came back.