''Teach comparison'' .
Luffy only reacted differently about it because Jinbe mentioned about Fleet Admiralship battle between Red Dog and Aokiji then Luffy reacted to it naturally.
Thats why Luffy asked Jinbe if Aokiji died or not after.
Because its a unique thing that 2 Admirals fighting each other, that never happened before and Luffy just learned that. Luffy probably never guess that happens while he probably knew BB was going to be very successful as a pirate since the day he met BB on Mock Town, look how Luffy is fully focused on BB when BB was talking, possibly knowing how BB will be his main rival:
So you are comparing an expected thing (BB's future as a pirate) to unexpected battle between 2 Admirals.
While Luffy and Red Dog never even speak to each other.
While Luffy sneaks into BB's conversation with Burgess on Dressrosa.
WB's final 2 opponents: Red Dog then fought 1 DF BB and died. WB's final villain was 1 DF BB not Red Dog.
Perhaps it will be like this, after Luffy beats Red Dog, Red Dog will show up somewhere else and fight Sabo instead fighting Marco, still BB should be the much stronger villain since even when BB had 1 DF Oda saved him for last vs. WB and get WB killed by BB not Red Dog.
-Red Dog's DF has the advantage vs. Sabo's DF.
-BB has the only DF ability to counter Luffy's Nika God DF.
Similar build ups.
Will Luffy still fight Red Dog?
Probably like WB's second final opponent maybe after Red Dog will change opponents like he changed to Marco, then it should be still Sabo's battle as well.
Luffy did everything he could about Ace, Sabo did nothing, plus Sabo has Ace's DF.
Revolutionaries wants to defeat Celestials, who are guarded by Admirals, Admirals are the only perfect opponents Revolutionaries can fight, fighting Pirates is Luffy's job.
Unless you believe Luffy should fight Marines and Revolutionaries should fight Pirates that would mess things up.
Since Oda said the One Piece story is for fighting against the Yonko, then the Revolutionaries only exists to fight Marines in the story which is why Sabo previously invaded Marijoa and fight 2 Admirals, and become the Flame Emperor.