Any reasonably sounding air to fuel raio can be safe or unsafe, it depends upon several other factors all together.

The above set of words, "knock from leaning out" is actually knock from too much timing while trying to make power. 12.5 - 1 AFR is where the power is, if too much timing then it will knock, so they und up using more fuel to lessen the power level. People would rather make less power with over rich fuel while keeping the timing too advanced because they lack the patients (and perhaps - skill) to do otherwise