Imo two things can be true at the same time here: DR Rocks can be high top (+) in terms of haki and at the same time Roger and Garp can still be (far) away from their primes.
DR Rocks, who in base might not even have any special abilities that make him tricky to fight such as FS or insane movement speed, is just mindlessly bulldozing here.
He's incredibly powerful, but not unreachable because of that. They can hit him just fine, if they work together, hence two non prime legends putting in everything they have and concluding this battle in such a way makes sense, even if he can sky splitt alone, while they are not even sky splitters yet.
Sky splitting is used as a signature for great pirates flexing and all of them only become exactly that - great pirates - years and years after GV, when they go and partially concluded their actual real journeys.
GV = Prime for anyone bar Rocks imo neither makes sense from a feat or from a lore perspective.