Keep in mind that the Death Guard are due getting a bit warp-lost, and things in the warp are rarely linear.
We also know that at least one Death Guard dude turned Daemon Prince was imprisoned on Pythos in the Pandorax system, which was also where they left Epimetheus. Pandorax was one of the first big missions for the Grey Knights after re-emerging. Then there's the thing about the Grey Knights actively trying to curtail Mortarion, which sees the elevation of Kaldor Draigo way in the future, and Mortarion tells him he knew Janus.
From the Mortarion's Heart script:
At this, Mortarion began to laugh. Draigo remained resolute.
MORTARION (snorting): \\Oh, noble and mighty Janus! Noble, mighty, steadfast, pious and honourable Janus–//
Draigo snapped his hand out in a punitive gesture, and a psychic blow flashed against Mortarion’s wards like the crack of a whip. It was delivered with the unwavering resolve of a beast-tamer bringing his charge to heel.
DRAIGO (helmet-vox): Do not speak his name. You are unworthy. Janus was the first of us to stand against the forces of Chaos, and the greatest.
The primarch still rippled with mirth, but shot through now with a flicker of agitation.
MORTARION: \\I am unworthy? I spit on the name of “Janus”. You are more like him than you could ever know...//
MORTARION (low, conspiratorial): \\But of course, I doubt even the archives of Titan would contain the original identities of the Sigillite’s... errant angels. Names. Power. It works both ways. Guarded. Hidden. [pause] The truth of Janus’s past would shake your pitiful Chapter to its very foundations. Of that, you can be certain. Treachery, cowardice... heresy, and a brother who would willingly betray his own for the sake of some half-imagined redemption.//
Draigo faltered, just for a second, but Mortarion saw the moment of doubt in the Grey Knight’s heart. The primarch’s words became no more than a filtered whisper through the rebreather.
MORTARION: \\As I told your master before I cut him down – you have all been deceived.//
So it makes perfect sense for Mortarion to be privy to the Grey Knights project. This was long coming. The question is how Swallow is going to handle it, and whether or not Malcador casts his weave in this novel, to hide Titan away.
Chances are we'll find out many juicy bits, I just hope they won't be wasted on another floppy attempt like Fear to Tread...