That would also mean his swords would all have to contain a large amount of iron in them in order to do that of course (even seemingly exotic ones like the Ultima Weapon), but then, this is a fantasy game – the laws of physics need not apply.
And incidentally, it appears that Zack’s starting weapon was actually the Enhanced Sword, a weapon that Cloud actually gets later in the original game – the appearance is pretty much the same. But I digress…