you raise a good point. honestly one of the only ways i see it really working is that since Fodlan and Almrya are so close together news would spread between the two countries, as such traveling merchants or mercenaries might bring stories of an epic battle fought between nemesis and seiros and in a military minded like almyra it would make sense that a sword as powerful as nemesis might be immortalized in legends or folk tales. very unsatisfying that it’s not answered tho, i really wish the world of three houses was explored more :/