It did seem like a ret-con though.
"No, he really meant to do all these other things, in this sequence, for his ultimate goal!"
Except that the whole "each Black Crusade had a specific goal" thing came directly from the fluff for Battlefleet Gothic almost a decade earlier, which specifically stated that even though the Imperium "won" that particular Crusade, Abaddon achieved his actual goal of obtaining the Blackstones. It wasn't a new idea, it just wasn't one fully presented in the mainline 40K
Please read the actual BFG
rulebook. Abaddon was aiming for all 6 Blackstones as they added their power together for ever more powerful effects, with 3 being enough to make a star go nova. After many years of fighting, he gets 3, and while going to trigger another nova, loses 1 back to the Imperium, and limps away with 2. Then all the remaining Blackstones in the Gothic sector, including the one recaptured by the Imperium, self-destruct. So out of an original goal of 6, Abaddon got 2 and then gave up on trying for the rest. Not exactly a resounding victory. Not a total failure either but hardly "Just as planned".