I can tell you straight up as someone who read most of End Times Nagash that Estalia and Tilea were destroyed by the Skaven offscreen. That in itself is a gigantic letdown worthy of reproach. However, Josh Reynolds was infamous for those responses, so take what he says with a grain of salt.
I believe he is the one who stated the whole "Settra defeats Kholek Suneater offscreen" thing that became a staple of community groupmind, but most of his responses were... controversial. Likely because he didn't run them by GW and was his personal opinion, but people took them as fact.
My understanding was that Reynolds was the individual who wrote some of the first, and most notably the last canon End Times novel, which IMO does somewhat elevate it from simply being an opinion. It's by no means word of god, but as I recall reading a really long wiki entry of his total responses, he acknowledges from the beginning that he's just submitting his best guesses or frameworks for what happened beyond the lens of the novels based on his expertise as director of both proverbial book-ends.
Of course, it's worth saying that there's a strong argument (underlined by this quest) that the Old World narrative would be best served by throwing the series, book ends included, out entirely.
Reading on Estalia and Tilea, it's incredibly disappointing for a number of reasons that they simply went under off-screen, particularly to the Skaven, who of the human realms they're indisputably most aware of given their proximity to Skavenblight. I would be far happy with a theory involving them getting blindsided by literally anyone else, but even given the power of proliferation of the Skaven across the entirety of the world, seeing the Dogs of War, Myrmidia and Sartosa made effectively into redshirts to develop their threat sticks pretty hard in my craw.
Given the mirroring of the Sartosa pirates and Dogs of War, it would have been much more interesting seeing them rolled into some sort of naval faction by the End Times crisis, so that there was some sort of significant Good naval faction to balance out the Evil ones (Dreadfleet, Dark Elves, Norscan raiders, indistinct underwater gribblies, etc.); reading through the world, it feels to a great degree that any other possible naval powers (Marienburg, Nordland, the Asur, Barak Varr) are largely overshadowed. It's understandable
why of course - they wanted to focus on the land battles, which is where the figs are - it's the same reason powerful hero mounts were constantly getting ganked, so they could have epic duels. But disappointing nonetheless.