Harald-Xebec was focused on more since the flashback is mostly Harald-centric. The conflict of Roger-Xebec is voided, along with any interactions that happened between Garp-Xebec.
It´s not like Oda was forced to do anything. He hasn´t exactly been restricted to focus on Harald and Loki this flashback, and is just using it as a way to connect various plotlines, about God Valley, Saint Shanks, Xebec etc. He decided the way to do this was making Harald Xebecs equal, who would have a close friendship with, because of a mutual respect of strength. Oda barely even attempted to justify their friendship beyond them respecting eachother on a strength basis after they clashed in Mariejois. We also know Oda wasnt even sure about actually doing an Elbaf arc, so he really did just decide on this recently, so he wasn´t pressured to justify something previously in the story or anything.
With Roger it's already implied they had met up with each other numerous times.
It's pretty clear from the below panel, Roger-Rocks were the two top dogs on the seas for piracy.
The narrator in the same flashback makes sure to remind the reader who the one that stands atop of the pirate world is. It's not like Roger was sitting on his ass doing nothing while Rocks and them were running around causing destruction. Blud became the 1st person in 800 years to reach lodestar, the 1st and only to ever do it even a decade later by the time of Oden flashback.
With Garp it's pretty clear that Xebec-Garp even before God Valley had gone against each other multiple times to the point Xebec was familiar with Garp's inner thinkings/ideology. Along with Garp actually showing active care about Rocks during their fight from being the 1st to ask Rocks about undoing his curse rather than just straight up whooping on him. To then being the one to go out of his way to comfort Xebec's feelings he was going through at that time.
While Harald-Xebec were friends on great terms with each other. It's made pretty clear to the readers multiple times in the flashback that Roger-Garp-Xebec have great deep connections with each other as enemies and have interacted with each other multiple times. His connection with Harald is given prominence since it's about how Harald was effected by Xebec. Showing casing Roger-Xebec-Garp interacting over the course of that flashback multiple times would've just increased the length of the flashback and took the focus off Harald, which God Valley did for few chapters.
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None of this is super meaningful tbh. Sure, they all knew eachother and viewed eachother as strong, but there´s a difference between that and the pretty clear dynamic between Harald and Xebec, an iconic clash, Xebec wanting Harald as his equal partner, making him clearly above WB for example, Xebec telling Harald his secret name etc it´s the standard equals duo dynamic Oda likes. Roger is the second top pirate of the era and Garp is the top marine of that era, but again, not super relevant. Harald wasn´t a pirate and was doing politics stuff, not going to pirate island and getting into fights. If Oda wanted to portray GArp or Roger as above Harald, he could have clearly had them fight Xebec 1v1 in an equal fashion, but he didn´t. He even could have had Xebec trying to get them to partner up with him, but again Oda didn´t do that.
And of course the below panel makes it clear who the 3-man dawgs from that era are. Harald isn't one of them, just like Newgate/Sengoku/etc aren't.
Yeah, Harald wasn´t made into a legend during the great age of pirates, so this isn´t really relevant? His era of doing crazy shit was pillaging the world 100 years ago, and he stopped after meeting Ida. After that, he did some secret work for the government, but it wasnt publically known to anyone, not even Elbaf.
And you can think Garp or whoever is equal to him, I don´t really care. My point is just that his standing at the top of the verse with Rocks is clear, if there´s more equals it doesn´t really change that, Oda is not going to have a scenario where he loses to Big Mom, because Oda doesn´t like characters losing 1v1s(not that is even the issue here, beyond powerscaling, average giants in Elbaf weren´t doing assignments for the WG, only Harald was, by himself. Seems like even Loki and Hajrudin don´t even know about his connection to the HK)