The size of a nuclear weapon in the hands of terrorists would matter less than where and how it was deployed. Had I no education in such matters I'd probably scoff the alarm this proliferation expert is sounding. Up until five or six years ago I'd have scoffed the suggestion airliners could bring down modern skyscrapers too.
I'd agree you might get into the 100's of thousands dead. But not millions. Not with 10kt weapon.
Like a autoimmune disease, the reaction to terrorism is worse than terrorism itself. After 9/11, the effect of the security measures and the fear of flying caused more damage to the airline industry than the initial attack.