I am really hoping it is hinting to something especially since, like you said, it is a lot of work for just an April Fools. It would be very interesting to see that Chaos forces were also tied up by Skaven.
Sure they are both Chaos but it isn't exactly the first time different factions have come to blows, it could be seen as an insult to have the Skaven pop up parking their city in the front lawn of your fortress and be seen as a challenge.
Also while people are waiting for Darkoath and Abraxia I recommend reading/ listening to Scourge of Fate. Half way through it at the moment but it does have Chaos warriors and Darkoath with the story being about one Chaos Warrior in particular on the path to becoming a Varanguard so a decent story to tie in with upcoming releases.