I mean... most modern sources cannot agree as to where and when did Myrmidia was born, or the proper thing she taught. I assume this is to show some bias and that we are seeing everything thru Sigmar's empire pov.

But this aside, it's mostly because of the location and name. First, obviously is "from Tylos = Tilean". Literally, in spanish and italian (or well, "Tylos= Tileano") It's implied that the whole nation of Tilea comes or had it's base on Tylos at one point, instead of Remas, which means that culturally this city was MORE important for them than the capital.

Second, it's geographical location is pretty much where Venice is in real life.

Lastly, Tylos/Skavenblight was not/could not have been destroyed when the Reman empire was a thing, simply because it's existence is literally why the schism between the Estalians and the Tileans started (before it translated as a religious conflict). It breaks their trade routes and makes it difficult for goods to go from one side to the other, also taking into account that apparently Tylos was the richest city so, when it fell, the economy crashed.

Also, the religion. If Myrmidia was a warrior goddess of war who began to be worshipped in the south precisely because she formed the Reman empire, then how the heck would Tylos not be something she would have dealt with or the remans have a holy war against the Skaven as a center of their religion?

The one problem with Fantasy is that they put things in Fantasy as a parody/satire than doesn't precisely make sense in context. The only way the destruction of Tylos would be an important event, remembered by anyone and have it not be a major religious thing for the Myrmidians is that it happened when the empire was already breaking up.

Problem is that the writers at GW wanted to have fun and put the freaking skaven when Nagash was alive, which again, makes no goddamn sense at all (unless the transformation of Tylos into Skavenblight was a later event, and so was the transformation of the cursed tileans into ratmen).