I had a look at the wind block on EK409's OFP. The flight preceding 8501. Forecast for RAFIS:
FL350 101/17
FL380 103/20
Hence he was very slow indeed.
GS of UAE409 was 503kts on the radar picture. This is a tailwind component of about 40-50 kts.
353kts as a GS would be about 300kts TAS at FL363, certainly less than minimum clean IAS. But then it is an Airbus.