There is a contradiction here. You cant say admirals are the strongest if Rox murdered one and by the wording of it before he made a name for himself on the seas and dying to Roger.
If Oda thinks the only way to hype the greatest pirate ever is to have him beat a nameless Admiral, something no other pirate has ever even come close to achieving, then yes Rox is at the top of the verse and so are the Admirals. You can’t sit here and pretend like Oda giving Kaido the likes of Hanafuda and Moriah but then giving Rox an Admiral doesn’t imply that Rox is massively stronger than Kaido.
Also keep in mind that Roger and Garp double teamed this dude. Bro beats one Admiral and the second greatest criminal scum has to team up with the Marine hero to fight him.
The portrayal is clear, imagine thinking Roger and Garp ganging up on someone who beat an Admiral isn’t direct Admiral glaze. Lol