As Sigmar founded the Empire, the Empire s closely linked to Sigmar. However, as SOF and I myself indicated, state troops bear the coat of arms of their liege, their immediate lord, which in the case of Talabheim is not the Emperor. Well, not since the collapse of the Ottilian Empire, which would not have used Sigmarian symbols.
That said, in the HRE, the Reichsstädte (free imperal cities) used the Imperial Crown above the shield to indicate that they were only subordinate to the Holy Roman Emperor. However, that would not usually show up in the livery or banners of their troops, and (depending on the edition), Talabheim is an Electoral Grand Duchy, anyway.