I'm sympathetic to your skepticism. A couple things, though. Public speaking competitions probably aren't the best contexts to gather evidence on this because the competition format dictates so much status. The competitors, at least in this domain, are almost always lower status than the judges. The whole point is to get the judges to like them and their speech, obviously a competitor signaling higher status than the judge isn't going to succeed. But I was involved in high school debate where status between debaters was routinely signaled by one debater treating the other informally and this included a relaxed posture. When interacting the the judges, however, behavior was more formal and the debaters would stand at attention (which is what I think the OP really means by stand straight). That said, the posture thing isn't the best signal because standing straight also tends to indicate higher socio-economic class, which in most domains is a high-status property.

Some high-status people could get away with it, but they'd be high status despite, not because of this activity.

But it still signals status because everyone who didn't know you were high-status before know sees that you're high-status enough to get away with blowing your nose in public. It is getting away with these things that is high status, not just doing them.