That's where it gets a bit interesting. See, that one guy is himself desperate to track down the source. Apparently he'd brought it up with Andy Law who himself mentioned it, but it was during a livestream and couldn't ask for details. It's another dwarf revolver situation.
2e's Tome of Salvation, page 17, has Myrmidia incarnate leading a united Tilea and Estalia 'as our glorious Empire expanded to the north and Sigmar's cult grew in power'. 'As Sigmar's cult grew in powder' gives us an earliest date of post-51, after Sigmar left, and more likely post-73, after the first Grand Theogonist founded the Cult of Sigmar proper. That already conflicts with the 20-60 date I've seen cited for Myrmidia's rule. 'As our glorious Empire expanded to the north' seems to be indicating the 'Drive to the Frontiers' era, which was 400-900. Combine that with page 40 describing the events of Myrmidia's incarnation as having taken place 'over two thousand years ago', with the book being set in the 2520s at the earliest, and you get hemmed in to the 400-520ish period. Sigismund the Conquerer's reign was from 479 to 505, so we are now at a point where if you're writing a quest set in the 400s, you have a set of lore justifications for Myrmidia Incarnate and Sigismund the Conquerer coexisting, which is the sort of dynamic I could easily see a QM being very interested in.
If you put a gun to my head, I could put together an argument that says that, well, the Cult of Sigmar
could be said to have started immediately after he left and the Empire
pretty much was expanding all the way from year 1 to about 1000, so
technically that source doesn't
explicitly rule out the 20-60 date for the Myrmidian Empire. But that would seem to me to be a deliberate misreading of the text.
The other potential sticking point is that the person in question does often try to add the Border Princes, and sometimes parts of the southern Empire, to a theoretical map of Tilea's greatest extent, and that's hard to square with Sigismund's expansionism. The key phrases from Tome of Salvation are that Sigismund 'led armies across the Grey and Black Mountains to found new provinces outside the Reik Basin', and that by 900 the Empire ruled 'a large part of the Border Princes'. You could square that rather easily if you just say that Myrmidia had parts of the western Border Princes spreading out from Dark Maiden Pass (which itself is named after a figure from Myrmidia's life) and the Empire had parts spreading out from Black Fire Pass, which would mean they could gobble up their respective halves of the Border Princes in peace without ever coming into conflict with each other, but that might not jive with attempts at maximizing Tilea's claims on the Old World.
Interesting, it doesn't seem like the province of Lichtenberg is ever actually named in Tome of Salvation, it's from Sigmar's Heirs. To me, that would support the idea that Sigismund never fully conquered the place, and maybe it got absorbed bit by bit over the centuries after him. Arguably, after the death of an incarnate Myrmidia and the disintegration of her own empire.
The
real sticking point is trying to hammer together a single set of hard facts at all. As we're all extremely aware, sourcebooks for Warhammer disagree with each other on objective fact all the time, and apart from the single 'over two thousand years ago' line above, all of the above was taken from in-universe texts, adding an entire second layer of narrator unreliability. I don't think we're supposed to be able to extract hard answers from this material, and I'm not even convinced that there's hard answers to extract in the first place. Sometimes I suspect that Myrmidia
never actually had a second incarnation, and that the whole 'Myrmidian Empire' thing is a misunderstanding or projection of the involvement of the Cult of Myrmidia in a otherwise mundane expansionist period, possibly tangled up with Empire sources tip-toing around Tylos and therefore the Skaven being inextricably involved in Myrmidia's first (only?) incarnation.
I think engaging with the setting means you have to be okay with ambiguity sometimes. The Warhammer wiki is inconsistency okay at having 'Canon Conflict' sections on some pages, which indicates at least some ability to let parallel ideas coexist; maybe an ability to let different historical theories coexist should also be cultivated. There's a lot of matters in our own history that we're unsure about, that we have conflicting theories about, and that's with a lot better archaeology and historiography to work with than the scholars of the Old World have. I think that the existence of multiple possible timelines for Myrmidia's Empire, and possibly even a question mark about whether it actually existed at all, is something that I think is a feature of what makes the setting compelling, not a bug.