That doesn't make any sense. Source please?
Um... The podcast. Look, sheathing and unsheathing weapons aren't relevant to the sysem. You have to think of spells as if they were physical weapons.
Look, there are two slots for combat activities: Left hand and right hand. In those hands you can equip one handed weapons, two handed weapons, and spells. The spells take up a slot just like the weapons do. So if you sheathe your two swords, those slots will still be taken up, and you have to actually unequip them and equip a spell to cast something.