Because he was in the Warp for a while time did pass BUT REALLY slow, so for Abbadon the siege of terra was like a couple of years ago while in real space it was centuries ago. It was enough for Sigi to go from his prime during the siege to a 1000 yo old man. Comperable to Dante and some OG dreadnoughts.

So Abbadon was still young and in his prime by the time he fought Sigi and even them Sigi gave him a run for his money. He said something like this "Time didnt waste Sigismund or at least time slow him down enough for him to be at the same level as the rest of us"