Ferrus Manus took nearly all his Morlock Terminators, alongside a hefty portion of other Iron Hands Astartes, no? That is why -perhaps- Terminator armour and Great Crusade/Heresy-era relics are so rare for the Chapter.
From what I can gather from Fulgrim, the Astartes not dropping down to Isstvan V, could have retreated to Imperial space, after the massacre upon the surface. Unless, they too were cut-down in Orbit by the four traitorous Legions, as theiir comrades died in the Dropsite Massacre?
Not too sure, really. I`ve heard Spikey* mention Raven Guard statistics on the Massacre, in an Imperial Armour Volume, maybe it`s there?