You're being too literal about the word psychopath. The original article claims, in essence, that war creates an environment where ordinary people can kill without it affecting them much personally. That is, from a behavioral point of view, what psychopaths do in regular society.

The article definitely isn't claiming that the 5% of so of humanity that is genetically incapable of feeling (much) empathy (i.e. sociopaths) make up 80% of the military corpus. The math obviously doesn't work out.

The article claims that war changes people. Some end up with PTSD, other people cope in another other way that makes them look like psychopaths.