Buck Converter Max Current
The powers and currents quoted are maximums, the most restrictive applies. It may also be the case that the maximum quoted can only be achieved under certain other conditions, for instance a particular input voltage, or cooling.

They quote 12A output current. That's likely to be the current that the inductor can handle. It doesn't matter whether the output voltage is 0.8v or 28v, the output current will be limited to 12A.

For the three scenarios you ask about, the most restrictive limits are
12v out - 12A current limit
9v out - 12A current limit
6v out - 12A current limit

It's not until you get to above 25v output that the 300W power limit comes into play.