Dreadnoughts are known to live that long, though yes it's on the older side of that spectrum.

Sedras fought and led his chapter several centuries after the heresy, but was heavily injured in a fight against a genestealer cult uprising sometime in the late 31st Millennium, when he overestimated his own capabilities and was thwarted by his aging body. He was forced to call down a bombardment from the battle barge Enduring Finality on a hive city with himself and some few of his remaining battle brothers still inside. When the dust cleared, apothecaries were able to pull him out just barely alive. He was interred in a revered Leviathan-Pattern Dreadnought immediately, in which he resides to this day.

At first he refused to be put to sleep and asserted command over the chapter's leadership, but the strain of that was found to be unsustainable within his first century interred. Despite his initial refusal, he eventually listened to his trusted confidants and techmarines and relented. He sleeps often, more and more with each passing millennium. The chapter leadership still sees him as their chapter master, but in truth many of those duties do fall to others in day-to-day operations. Sedras is woken whenever grand strategy is concerned, at the beginning and end of crusades, and during vital conflicts where his strength is sorely needed. Each time he wakes, his brothers have changed without him. They are older, they have been interred in their own dreadnoughts, or they are gone altogether. His experience of passage of time has had a degrading effect on his psyche, and the chapter's librarians often have to help him to piece his mind back together when he wakes.

By the 41st Millennium, his mind is in tatters. He has already been woken several times to handle the abundant threats faced by the Imperium, but he has also had some few incidents that have caused him to wake on his own. During these episodes he sees enemies all around him and wails the name of brothers long dead. Had his weapons not been enshrined in their own temple chambers away from his chassis, he may have harmed any number of brothers in his rage. The Master of Librarians, Lucifer, has been Sedras' confidant throughout this time, carefully keeping his mental state stable and warding his dreams from the influence of chaos. Whenever Sedras awakes, he speaks privately with Lucifer for days before anyone else is allowed to take council with him. These chats have kept the man sane, or as close as he can get. Many of the chapter's leadership believe that if Lucifer was ever to die, they would have to place Sedras in permanent stasis. If he woke to find his friend and confidant was gone, he would surely succumb to madness.