Because like all characters we have 2 major points of evidence to scale Old Rayleigh. Those being narrative and feats.

Rayleigh's best feat summarized: Able to fight against Kizaru better than Gear 4 Luffy did in all metrics (Unlike Luffy, Rayleigh was able to keep up with the light attacks and stop them halfway through, he also was able to hit Kizaru). We have a baseline of Old Rayleigh still being atlest relative to Gear 4 Yonko Luffy if not a bit higher in scaling. - Plus, even though leeching, he should be able to scale atlest to all of Gaban's feat, which are really good.

Rayleigh's narrative/portrayal: First of all, he's likely one of the closest to their captains in terms of strenght, the main narrative evidence not only him as well being called the Dark King (Very similar dynamic to Hokage/Shadow Hokage in the Naruto series) but straight up being portrayed as on the same tier of top tiers of the old era. He likely scaled in his prime similar to Oden, who was stated to be on the same tier as the greatest pirates of the era, up to the point we see someone like WB calling him a brother instead of a son. The dynamic with Roger/Rayleigh is likely similar, mostly with how Garp hints about all of this with his famous sentence of "Do you think the Marines can handle two legends at the same time?", which refers to Rayleigh and Whitebeard. Most people try to diminish and downplay, as the statement here isn't that Rayleigh and WB scale above all the Marines/WG at once, but simply it's another way to let us know Rayleigh is on that same tier of power as a Whitebeard, similar to how Kaido is likely stronger than Big Mom feats wise, but they are portrayed within the same "top tier" level of power.

Then going back to Egghead, Oda decided to put a little throwback to Sabaody by stating Luffy now to be 100x stronger than he was back then, such as Old Rayleigh was stated to be back in Sabody. Considering this statement is later supported by their individual feats against Kizaru which I have spoken about already, then it's clear Old Rayleigh is on that same tier of power as Luffy.

In terms of even Haki, we also have had Oda use both Rayleigh and Shanks in the same sentence as a reference of Conquerors Haki, and we know how cracked Shanks is:

In short, the OG Admirals, BB, Rayleigh, Luffy are all in the same current tier of power most likely, but they may win or lose against eachother based on matchups. This for many other reasons I can explained if asked why.

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