Good faith or not, there is no faith assumption with telling folks to shut up. Its rude. The onus is on the poster, not the recipient to ensure messages are not ambiguous.
It's only rude if it's intended to be, otherwise it's just words. He was just exaggerating his advice is order to make his point clearer. It's impossible (or, at least, impractical) to ensure messages are completely unambiguous, so the onus has to be on the recipient to try and work out what the poster intended and when doing that you should always assume good faith.