That all makes sense...until Fafnnr says, "Not so undeserved a reputation after all, eh?" What sort of reputation are they deserving of now, suddenly, that they would not have deserved after annihilating two other Legions 50 years before?I don't see how this is that ambiguous. Faffnr references the rumored duty for which his Legion is so notorious, and Guilliman knocks him down a peg. He's essentially saying, "You're not that special for doing what you claim; we all do what me must, according to our ability.
Also it is incredibly unlikely the two lost Legions met their ends in two separate tragedies.