723.M36-736.M36 — Free Captain Haarlock undertakes the Great Voyage, which discovers the Calyx Expanse
945.M36 — The Nemesis Incident
322.M39 — Angevin Crusade begins
The Storm Warden homeworld is given as Sacris, in the Calixis Sector and wasn't described as having changed during the Nemesis Incident. So is it a case the Storm Wardens were only based on Sacris for a century? Chapters have been known to change their homeworlds (if rarely) and a recently discovered expanse may have drawn the attention of the chapter, whether they were new or old. On the idea that the Storm Wardens were a relatively new chapter, the Cursed Founding was also in M.36. Perhaps the Storm Wardens are a lesser known branch of that founding, which would tie into the theory they are loyalist World Eaters (not my thinking, but it is possible).
There is also the question of how much the Deathwatch knows of the chapter, assuming the Deathwatch would be one of the first Astartes groups to likely begin operations in Calyx and to have known about it before Haarlock officially found it. Perhaps some watch station or fortress within Calixis holds the key to unravelling the Nemesis Incident?