"Not quite how it works. Ah, but before I move on to that, it seems prudent to include any warnings about things they warned us really hard about, so..."
Civilization emphasizes really hard to kids at this point that, when you reject a 7:5 split with probability <6/7, you're not trying to spitefully punish the person, just make sure that their incentive curve slopes slightly downward as it moves away from what you think is fair. If you were trying to spite them in accordance with base instinct, you'd reject with probability a bit greater than 5/7, so that they lost almost as much as they tried to gain at your expense (even spiteful entities, obviously, will still subtract epsilon from their spiteful punishments to avoid the possibility of infinite resonating spitefights that even they don't want).
Keltham has no particular reason to think Chelaxians are likely to make that particular error, but dath ilan emphasizes it hard to children, so it's probably important or a plausible error that somebody might otherwise make.