I wouldn't necessarily say that. I think all 3 are pretty even Luffy/Ace/Sabo, it is just Ace and Luffy are soo damn similar in this flashback and also factor in we just are getting introduced to Sabo first time here it seems like Sabo is getting mroe attention.
I mean imagine if we didn't know who Ace's father was and that was mentioned in this flashback. It would then be equal. but we already knew half of Ace's quirks and his family. We find otu more that Ace is like basically Luffy in their goofing off and all that. Sabo is thier buddy and goofs off too, but isn't as you say "Two peas in a pod" like luffy and ace are. And we learn of sabo's family, which we already knew about Luffy adn Ace's family.
So the three background is pretty much equal, sabo just has no present
IMO, Luffy gets a lot more than the other two. Unlike Sabo and Ace, you can clearly see Luffy's quirks in his childhood. He's gulliable, can't lie, cries a lot, has weird ideas like the gomu gomu attacks, is still not a badass…
Sabo and Ace behave "seriously" (still like children, but without such personality gimmicks) all the time and don't have quirks like Luffy. You can clearly see child!Luffy as an undeveloped and childish form of his present self, but Ace is pretty much the same he will be as an adult. Sabo is a cool character, and last chapter made me like him a lot more, but he's still not shown a lot of differences from Ace besides in background and temper.
That said, I must confess this later chapter adds an interesting possibility for Sabo, which is joining the revolutionaries. He could escape with Dragon and later appear in the story. There's still the possibility that him betraying the nobles leads to his death, but at least now we know he met Dragon.