A lot of theories suggest that Whitebeard was still in First Commander Level and not yet a Yonko Level threat, that’s why Garp and Roger could’ve been successful in God Valley.
Still even then I don’t think Roger and Garp alone could beat Rocks D. Xebec and that whole crew including Whitebeard, Kaido, Big Mom, Shiki, etc.
Remember Shiki and his crew seem to have almost killed Roger in his prime, that’s how strong him and his crew alone were. I also believe all the future Yonkos were already starting to show their prime potential powers, that’s why Rocks assembled them and formed that crew just 2 years before that final fight.
So how could Garp with his marines and Roger and his still young crew possibly beat this super pirate alliance?
Chapter 966 shows how Whitebeard and Roger seem super friendly despite them being rivals and former enemies. I believe this is because both Whitebeard and Roger fought together with Garp to bring down the Rocks Crew.
Now, I hear you guys, “What? Why would Whitebeard even do that?”
First, for Whitebeard nothing is bigger than his family. Rocks has been described as an incredibly vicious and ambitious man, having no care as to who he attacked as long as he could garner power for himself and leading his crew to destroy whoever interfered with his ambitions.
This leads us to believe that most likely Rocks was gonna start a plan that would kill his own men, which would’ve been flat out unthinkable for Whitebeard.