Many possibilities, and I've no Airbus knowledge, so these are general comments.
It depends on what the engine problems were. If they thought they could return to Detroit, but then decided to land much earlier, that would suggest they knew they had a substantial problem, but not yet a total failure. Some would have planned the whole approach and landing as an engine-out manoeuvre. Others, as you suggest, might use both but brief & plan for the worse. (If they were operating the problem engine at reduced thrust and then idle during descent, perhaps they thought it was more robust that it proved.)
Some airlines have an SOP of GA if an engine failure on finals below XYZ feet. Others allow captain's discretion to continue. If the former, then planning and executing the approach as an engine-out, even if only at idle, would remove that possibility: i.e. no surprises. One wonders what their ELW was. That could have led them to hope for 2 engines.
What happens on an A330 if you fly as if SE, but one is at idle? Does that confuse all the computers and cause more problems?