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In 1981, the Soviet Union purportedly unleashed a fungal chemical weapon dubbed "Yellow Rain." The Wikipedia article has the basic info, but here is a bit more:

I wasn't trying to claim Russia would cut itself off using this old Soviet weapon, but sharing something similar to what Irlmaier described. Admittedly, there are a lot of problems with the comparison, because it is not thought possible for a fungal chemical weapon to operate as quickly as described by Irlmaier's "yellow dust" weapon. He might have actually been referring to a more effective nerve agent, but of course, we can't really know. Some supposed (unverifiable) whistleblowers have claimed that the U.S. government attempted to reverse engineer the technology of the Soviet Union, including that of the gravely immoral type. It could be that the U.S. government secretly possesses a better version through these past efforts.

In agreement with the first two sentences. As for the rest, the neutron bomb part doesn't fit with "yellow dust" and a line of death that causes people to be "quite dark." Instant death and blackened (?) bodies only really fits with nuclear detonations when it comes to CBRN, but both the delivery system and line of death description fits better with a chemical weapon over a radiological one. I think we come to the same conclusion with your final clause concerning chemical agents.

Irlmaier's "quite black" and "withered and black" present difficulties if we take what he says literally. The weapon apparently kills instantaneously, can be spread along a line, seemingly does not spread from the line once dispersed, and causes both humans and other living things to blacken in appearance.