The point is that people are going to see that the post was edited, because most platforms will tell them, and the poster is saying “yeah, it’s edited. Don’t worry, the meaning hasn’t changed”.

Asking how you’d tell if they were lying is really missing the point. It’s not evidence being presented in a court of law, it’s social etiquette.

Handshakes date from a time when the person you’re meeting having a knife they intend to stab you with was a serious concern, so the custom of grasping each others dominant hand to say “look, I’m not holding a knife” became popular. Doesn’t stop people from having a weapon in their other hand, but would you say handshakes are pointless?