The "magic numbers" are more like an observed pattern, not a real constrain of the universe.
What's interesting for me however is that they detected four neutrons, instead of detecting silicon. Si-28 is extremely stable, and it could be in theory produced from O-28 through beta decay. This means that the repulsion there was so strong that the nucleus "decided" to eject the offensive particles instead of working with them.
One way to interpret this is that neutrons repel neutrons through strong force, but attract protons. So if you got enough protons you're able to glue some neutrons together, but if you got too few of them your atom will shit neutrons. (Note that protons hate each other twice - because of strong force and electrical charge; so a hypothetical Ca-28 would be even less stable than an O-28.)