Both of these predict higher status.

Evidence? There are plenty of low-status people with bad manners or bad posture. What data do you have that leads you to believe that, for some X under discussion here, on observing X, you can systematically infer the doer of X is high status?

More precisely, not suffering sanctions and emotional resilience may predict higher status, but does the behaviour under discussion reliably predict those characteristics, or may it predict others? You ignore the fact that if you display low-status behaviour at a social gathering, people won't kill you and your offspring; you just won't get invited back. Thus, people can quite easily repeatedly take actions that lower their status. This is even more so the case if they are somewhat oblivious to social rules, and, of course, being oblivious to social rules makes them far more likely to act "selfishly" (as used by the original author) in the first place.

I believe you need to observe a rather complicated series of events to infer that any particular selfish behaviour shows high-status. This makes the concept advocated by the post's author rather useless: selfishness signals high status, except when it doesn't, which is quite frequent and hard to identify precisely.