Good point.
It's very clear that everyone around Godai is concerned he will become a purely destructive force, a "living weapon", the more Kuuga's power grows. Maybe this was people taking that to the logical conclusion, that if Kuuga is destruction, then his ultimate power, is ultimate destruction.
We never see the blast radius of the Amazing Mighty kick in that form's sole appearance, just a giant pillar of flame.
I'm actually googling this now, to try and see if there was something that established this as an official thing, that Kuuga can destroy a whole planet by kicking something hard enough... not been successful so far though.