My bad then. I thought he got sucked into the warp fighting M`Kar which the GW page said happened when he was low level.
Nah, basically it went:
Banished M'kar (as a battle-brother) on Acralem and was subsequently cursed by the Daemon > Rose through the ranks to Supreme Grand Master > Returned to Acralem where upon the curse took effect and he was dragged into the warp.
But even so your point still stands. I imagine it would still be a little difficult to coordinate the Chapter when he is wondering through the warp, outwitting Tzeentch and generally just being stronger than Khorne and better than Chaos...
Unless of course the title of Supreme Grandmaster
is now more of a respectful titular role (without holding any direct leadership role), not bestowing it on a successor considering Draigo is still (somehow) alive (in a sense).