The weapons are actually derived from the 1970s and 80s research into neutron bombs. The reason the primaries are compressed and pumped via external neutron sources is to greatly reduce the amount of fission fuel - meaning very little gets vaporized in the blast (so little that the ground area under the blast is safe after a few hours), and almost all of the energy comes from lithium-6 deuteride fusion. The blasts are also relatively small - a few kilotons (about a third of a Nagasaki) and are meant to go off thousands of feet in the air, safely away from the ground and high enough to not start fires. While the neutron pulse is slowed by an external casing of moderator (beryllium lined graphite AFAIK) to reduce the range and potency of the neutron radiation output. There would still be some significant compton scattering and so a modest EMP would be present, but we're talking a choice of "oh no my iphone got fried" vs "oh hey that Chinese ICBM with a 4MT warhead just blew up in my face" the choice is pretty clear.