This internet version of centrism that everyone hates on is bizarre to me, for the reasons you say - the “only commit half a genocide” type of centrism. Are there people who really strike a middle ground on every issue on principle?

That's not a principle, it's just a rhetoric. Like when people demonstrated in Charlottesville : one side wanted to get rid of a slavers statue while the other side run a car through the crowd. Then Trump took a "centrist" position and said that there was some very fine people on both side, which was just a way to defend his side