Hello,
I've been trying to use Overpass Turbo to export geometric shapes for these relations with "type=land_area" tag. However, it only results many segmented lines, rather than a single, combined and complete multipolygon. 
From my observations, it seems that it's only possible when they are tagged with "type=boundary / multipolygon". That's why I modified them.
Please let me know if there's any alternative way to export these relations as complete multipolygons.