"Shout quietly!" sounds like something you would hear and pause for a second. How is that possible? But it make sense the more you think about it. Not everything calls for noise. Sometimes the most audible thing you can do is to remain still and still be heard.
It's like when you say something important without raising your voice yet everyone listens. Meaning without drama, without extra energy. That has a different form of strength. You simply understand that what you are saying matters; you are not trying to prove anything.
Though not always the case, people sometimes think being loud is the only way to stand out. Especially when it's truthful, a little voice may have great force. It earns attention; it does not fight for it.
Therefore, "shout quietly" is not exactly shouting. It's about speaking powerfully without volume requirements.