Karhedron wrote:GW love to tease us with little hints like this. It is definitely deliberate but until it gets confirmed in canon, treat it as amusing speculation.
The closest we get to answer is "The Room at the end of memory" which implies both the 2nd and 11th Primarchs were killed but that Marines from both Legions were inducted into the Ultramarines and Imperial Fists.
I have heard of the Ultramarines getting one of the fallen legions, even as in universe speculation, but not the Imperial Fists.
However, I made the connection to the Soul Drinkers. They believed they were second founding Imperial Fists, but it was later shown their gene seed wasn't from Dorn. Unless the newest book(s) after "Phalanx" cleared it up, their original legion was unknown. If the Imperial Fists did get marines from one of the lost legions, and one of the second founding chapters was made up of these marines, then the Soul Drinkers could both be Imperial Fists, but not of their gene stock.
Just a random connection I made, with no proof behind it. But it does slot in real nice, but only if the Fists got some more bois.