The problem would be in claiming Jaina had the power to do that, absent the use of the Focusing Iris, etc. etc. Illidan is plenty powerful, but neither he nor Jaina *alone* could be considered planet-killers - that level of power is beyond either of them. Give Khadgar the Focusing Iris, or the Sargerite Keystone, as well as a reason to kill a planet and you'd likely end up with the same results. The power here is mostly coming from the intensely powerful relics being used, though; not the person using them.