Partly it's a judgment call. I would say that bringing Churchill into a thread about English writing style (which was already off topic, but not a flamewar) does not count as flamebait, while bringing in genocide does. I can see how someone would argue it the other way.
Mostly, though, it's a matter of the effect on the thread. The value of a comment is the expected value of the subthread it spawns [1]. In this particular community a "Churchill+genocide" comment is pretty well guaranteed to spawn a flamewar, which was not true of the subthread one level up. So it's relative to the community you're participating in.
[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...