You don't know that. All we know is it dropped for a little while. Once they are out of radar range, given the level of planning demonstrated, I'm sure they went back up to altitude.
Do don't have this level of planning and fail to figure in range. No need to fly low over the Indian Ocean where no radar can see you.
Don't know what you base such an assertion upon; but, never-the-less, true airspeed at lower altitudes is less and fuel consumption is greater (as well as in the climb) - all of which diminishes the overall efficiency of the jet, thereby - in this case - lessening its range.