Imagine if this was true.
I think Xebec was better than Roger. And he was actually morally even better. The whole reason the WG calls him a demon, or evil or trying to erase him from history is the SAME REASON, they wiped out Ohara people, and they called Nico Robin, an 8 year old kid as "demons child"
Roger in my opinion 38 years ago knew NOTHING about the void century. While, Xebec was invistigating in that stuff. Roger only realized the existance of Poneglyphs after he finished his FIRST JOURNEY, which I can safely assume after Xebec was fallen. And then, he scouted Oden out to actually recruit him and with the second journey, they read the poneglyphs, and knew the entire history.
However, when Xebec was alive, he was ahead of Roger in all of those things. And I personally think that Garp wouldn't hate to talk about the incident if the reason for it simply is Roger didn't get enough credit for it or he cooperated with a pirate. Rather, he would have been like Smoker in Alabasta telling the world that Roger had a hand in taking them down as much as he did. Just like Smoker, he wouldn't want to get someone else's credit or be owed to them
So, it makes no sense if that was the reason for Garp hating to talk about it.
The real reason was actually them siding with the Celestial Dragons (who had slaves in that event).
I think that Garp simply followed his duty as a marine to protect the Celestial Dragons that night.
While Roger was actually not concern whatsoever about those things, he was just too occupied and too hotheaded that he wanted to beat Xebec, doesn't matter who was right, he just wanted to get a W against him since he was his biggest rival according to Sengoku.
however, I think during the fall of xebec, something triggered Roger to follow his path which leads us to Roger going BEYOND and knowing the entire history, BUT because of his illness, he didn't have power to do anything about it. Leaving it to Luffy who will surpass those two Xebec and Roger, and go beyond and DO something about it