I agree... Roger just didnt want Xebec to go to raftel or be crown as greatest pirates ever
So he teamed up with Garp who had a duty to fulfill
Nah, I suspect neither Roger nor Garp knew anything about Xebec's end goal.
Roger was simply motivated by his reckless behaviour of fighting him as a rival
Garp was motivated by the fact he is a pirate and needs to be stopped as is duty as marine.
And I think a legendary person like Xebec who can be put on the same pedestol as Roger, WB, Garp...etc. He would not actually try to make excuses to postpone their fights and clash. He just is too proud to not accept their fight when it happens.
Point is, I think there is no denying that Sengoku admitted that Garp hates talking about that event.
And there is no way the reason for it is that he teamed with Roger, in fact, he would be like Smoker with Crocodile storyline, telling the world government that Luffy took down Crocodile not him, and Garp will at least be a talkative perso about it giving credit to Roger.
But hating to talk about it? I'm sure that Garp witnessed something bad and shady about the side he was fighting for. Or he feels somewhat ashamed for stopping him because he feels Xebec had a point. THERE HAS TO BE SOMETHING