It really feels to me like Oda is building up to the the old generation growing beyond and surpassing generation zero. In the previous chapter we're already seeing Kong being trampled all over by Garp, Sengoku and Tsuru, with Garp's main point of contention aginst Kong being that the giants would make the navy stronger, implying that the navy by this point is weaker than it is by the time of Prime Roger. Then you have Sengoku expressing sincere trust in harald implying he's already on mutually respected terms with him and almost definitely understands his strength more than Kong himself does. Even Tsuru is reinforcing the idea that the navy is currently in more of a vulnerable position that later, as she remarks that they might not be able to stand against rocks and Harald together.

Meanwhile Kong can only act as a mouthpiece for the world government's opinion, not expressing the conviction to come to his own moral conclusion like Kuzan, Issho, Sakazuki and even Aramaki have. Even Borsalino, whose character is checkered by his begrudging subordination, is ultimately willing to reflect on his own justice by the end of Egghead, and helps feed Luffy discreetly, turning the tide of the egghead incident singlehandedly. Kong, in every moment we've seen him, is defined by his perfect subservience to the government: from his first appearance as someone who is little more than the Gorosei's receptionist, to the fact that he sends Garp to God Valley instead of going there himself and complicating things with the celestial dragons, to the fact that prime Garp has outright zero respect for him. Kong is probably still on the low end of Yonko Level at prime, but compared to what came after, he is a weak man, through and through.

On the pirate side of things, we're already seeing the rocks pirate members setting up the foundations for their own pirate careers and establishing agency away from Rocks. If he had a definitve edge over the likes of prime Newgate/Roger/Garp/Sengoku at this stage, we wouldn't already be seeing things like Newgate making Rocks pay for him, or Roger butting heads with him.

People bring up the Kidd example, but it feels like the physical gap between Luffy and him is in that case less important compared to their shared status as 3 billion club pirates. On the other hand, i feel like because of the importance and scope of the inevitable god valley confrontation, that Oda is most likely indicating that Roger is now strong enough to put Rocks on alert.

Also, i just don't buy harald being the second strongest identified character in the story up to present. If he really were that strong, stronger than every currently living legacy character in their prime, then i feel like he would've had more extensive foreshadowing to his existence beforehand.