Horse manure.
Making a statement like "setting off a Hiroshima-size weapon could cause as many direct fatalities as all of World War II" exposes these people as idiots.
If the problem is fire and smoke, the firebombings of cities in Europe and Japan during WWII often exceeded the fires from the nuclear drops on Japan. The climate didn't change. Nor does it change significantly from massive forest fires every year.
Incidentally, Pakistan and India between them don't have more than 60 or so warheads all together, and even if they had 50 each, the likelihood of both of them exhausting their entire stockpile on each other is nil.