As you point out, the increased ultrafine particles per mass of fuel in improved stoves is offset by the decreased mass, resulting in no significant change there, but a major reduction in overall particles and saving of fuel cost.
If the increased efficiency of the stoves means they are used more, that will only be to give those households a better standard of living (warmth and such). That
should be discouraged? What other means would they have to stay warm, and how much pollution would be created in producing it?
They say "study... found that designers could cut black
carbon emissions from stoves using forced drafting." Yes, that sounds like a reasonable solution for millions (billions?) of households that don't have any electricity...