This is not the interpretation of Thumperism that I was taught. I was taught it as a “don’t hurt people’s feelings with critical comments, whether solicited or unsolicited.
Unsolicited example: “Kids, don’t say ‘Wow, your breath stinks!’ Just keep that to yourself.”
Solicited. Soooo much trickier: “Girls, if someone asks if you like their dress, don’t say no, find something nice you can say such as, ‘I love the color.’”
No one ever told me not to defend myself verbally, just that I wasn’t ever supposed to lay into anyone for something they couldn’t do anything about. This means no criticizing physical appearance or intelligence. Character and behavior was fair game. I think that is a very good rule.