I have flown with this box before and I have made the same mistake in failing to flip it from GPS to the ILS.
The report does state that in the absence of a GS, as indicated by the note on the chart the pilot may have elected to descend on a LOC only approach.
That wasn't the issue, it was using the wrong reference point for the distance, using the NDB ( GPS distance from the NDB )instead of the ILS DME, the descent was premature.
