Reuters published this statement from DGAC: "The aircraft did not itself make a distress call but it was the combination of the loss of radio contact and the aircraft’s descent which led the controller to implement the distress phase."
It's hard to imagine the crew not making contact for the entire 8 minute descent unless they were severely impaired or somehow had an electrical failure leading to loss of all RMPs/transceivers.
Perhaps there was a slow decompression that left the crew somewhat impaired by the time it was clear there was a loss of cabin pressure leading to them selecting OP DES instead of donning their masks immediately.
If you listen to some of the ATC recordings of hypoxic pilots in the past you will understand just how much it can impair your thinking.