Then Mordheim is taking place soon after 2000 and the Empire in Flames stuff during the early 2000's, before the Empire was united again.
I think many aspects of the WFB world are repetitive: chaos incursions, orc incursions, civil wars, chaos/beastman/goblin outbreaks, dark elf piracy, dwarf expeditions, etc. etc. From the point of view of a single campaign or a thematic tournament, setting the exact date often is not important, and the game still catches the feeling of the milieu.
My games have usually featured a looming chaos incursion, a looming orc incursion, the presence of a growing undead/vampire threat, and lots of domestic troubles in the Empire. Apart from a couple of big names from around 2520, all of that could happen any time.
We could then split hairs about when gunpowder became effective and commonplace, when the Steam tanks were constructed, or when the landsknecht fashion became prominent in the Empire. We know how the timeline goes in Europe (try to fit the steam tanks somewhere there?), but it seems the Empire has kept revolving around these themes for a considerably longer time, yet the halberd is still a staple weapon and real pike & shotte warfare seems to remain in the future.
I think this stagnancy is some kind of a hallmark of fantasy. Maybe Tolkien started it. Maybe it's older and goes back to ancient beliefs about the sidhe/fairie/dead living their eternal lives in the otherworld or in purgatorio? Maybe there is a bit of longing for eternal stability there?
-Z