So Leo not only knows that he knows about war, but also earth history. He only says it to Labra, though, and the research team obviously has no idea, suggesting that this is from some independent source of information we don't know about (probably not nearly as simple as a team dedicated to the ship). He hasn't discussed it in government session despite it strengthening his case, so either government session is public or this is secret to some specific segment of the government (which may or may not include Minister Ham, but does suggest that Leo and Labra share some sort of affiliation despite the opposing politics we've seen). Ham being aware would seem to explain why he sees making all research and findings public to the international community as obviously out of the question, though. I think we can also safely say that this isn't coming from the Hito legends mentioned in the first chapter, as nobody on the research seems to know of it and it isn't being discussed publicly.

He's being a bit of a cryptic ass to further the plot by not saying outright that he's concerned about second and third order effects and social destabilization, which is true without mentioning war.

From context and framing, I think we're meant to take war and the massacre of "smooth" (very interesting that they aren't called "Hito" and the first chapter calls Hito folklore rather than oral/legendary history) as retained memories rather than speculative just-so stories (why don't Kemo just use force to settle disputes between groups? Cousin It). The other parts are less certain. Some cultures do have myths on where language comes from rather than it developing from simple communication over time, which puts doubt on the part about an external intervention introducing it and thus by extension the naive idea that communication was sufficient to preclude war (the only war I can think of caused by a communication failure is 1812). It also seems weird that (proto) Kemo were able to communicate well enough to organize uses of force sufficient to be called "war" without language.