I assume they mean that if you equip Simurgh (or its first upgrade) and four Kaiser Knuckles Hope becomes immune to physical damage. Except he can't be healed by magic, and the weapon offers no magic stats afaik...
Fang has a weapon like that too, but they're the only two.
Fang and Snow have a similar weapon that equipped with four Magistral Crests make them immune to magical damage, but also can't be healed by magic and have no strength stat.
I have no idea if there are any practical applications of either weapon in the endgame (since you only get the fourth Accessory slot after beating the main story).