It is very unlikely that 3 D alkaline cells can supply enough wattage for even the most efficient LED to generate 1000 lumens. It would be closer to 500-600 lumens.
They just quote the actual LED's capability, not OTF lumens. Even at 500-600 lumens these batteries will get eaten up in no time. Lithium primaries in D-cell holders will last as long, if not more and with less weight.