And if Sefar came again, why do you think Excalibur wouldn't be in planetary defense mode...?
Zelretch's Moon Cannon thing was explicitly the result of him using a Mystic Code, a product of intelligence, coupled with a special magic circle, a product of intelligence, to fire an "Ether Cannon". Sefar is dangerous because it absorbs anything that is the product of intelligence, no matter how basic or undeveloped it is.
Maybe you could argue that Zelretch's thing could work, even if I don't believe it would, but even then A) he can't use that kind of power anymore after his thing with Crimson Moon, and B) why would you assume he's capable of matching Excalibur's output when it's meant to be the strongest thing ever on the planet?