I asked osm2pgsql maintainers for support. Maybe this tagging should be changed. Maybe this relations shouldn't be used at all, as land areas can be constructed from boundary=administrative and coastlines (and I desparately miss a land area relation for the Netherlands).
For the meantime I use the attached query to get the polygons. Note that this has to be ...
You can add the instruction for boundaries
node,way boundary text linear
But since you have used the --hstore option, you could query it:
SELECT tags from public.planet_osm_polygon
WHERE tags @> '"boundary"=>"administrative"'::hstore;