In any normal social context it would be reasonable to assume that this an overconfident statement deliberately made without caveats in order to enhance bargaining power. Which is fine - humans are selfish.

That isn't true. There are times where overconfidence is used to enhance bargaining power. But people just really not liking people doing things that hurt them is just considered normal and healthy human behavior.

This being LW where there's a good chance that this was intended literally - this sort of rigidity was exactly why "learning how to lose" is a skill.

No, it isn't. Learning to lose is an independent skill to knowing what 'lose' means and not liking to lose.