Also, Kang said that Klingons had honored the peace treaty for three years in "Day of the Dove". Which shows that
1) Klingons were characteristically honor-bound in TOS already
2) Kang was a filthy liar
3) The three seasons probably covered the entire five-year mission after all - otherwise it wouldn't be possible to fit three years between "Errand" and "Day"
But yeah, it all looks like the Organians told the two warring sides to keep it down lest there be consequences, and they then kept it down (a few small-scale massacres or a proxy war here or there won't matter, surely) but upped the volume gradualy until they were back at the original noise levels, just to see if the Organians cared. And they did not. (Which was as it should, because the whole point of "Errand" was that the Organians were constitutionally incapable of caring.)
Timo Saloniemi