Malack was relatively low on cleric levels. with a cr of 20, maybe he felt he could not afford to spend a 6th level slot on word of recall. It's quite redundant anyway, since he can always escape in gas form. Being stuck in sunlight is the only time when word of recall would really be useful. But then, that would requiren someone to dispel it in the first place, something to prevent him from casting his second prepared slot, and something to prevent him from casting it from the staff. Pretty unlikely, I say. I think another heal or harm or windwalk slot would be a reasonable choice.