His character description in CFYOW verbatim says
“The Eleventh Company captain. His title, Kenpachi, befits him as he is the strongest Soul Reaper.”
Sure they spend their time saying title of Kenpachi = strongest Shinigami but here it essentially says “His title of Strongest Shinigami befits him as he is the strongest Shinigami.”
If the description didn’t exist, you’d have more of an argument but here we have Ikomikidomoe breaking out of Ichibei’s hax with raw power, something even Yhwach didn’t do, and Kenny one taps it.
“Even a small fragment of the Reio seemed to have given Ikomikidomoe immense power, to the point that the Manako Osho’s zanpaku-to ability had been weakened and he has apparently regained his true name that had been overwritten.”
“Ikomikidomoe turned toward the voice behind him, and …
… leaping high from the sky with Nozarashi, a gigantic zanpaku-to larger than he was tall, gripped in his hand, a demonic form filled Ikomikidome’s sight.
“I’m not letting anybody take you from me.”
Then a blow like a fierce god’s - a blow strong enough to smash a meteor - came down on Ikomikidomoe.
In the end, Ikomikidomoe was unable to say his true name.”
So even without the character description, Kenny would still be the strongest.
Shunsui < Shikai Yama < (x5-10) Bankai Yama < Ichibei
Shunsui = Shikai Byakuya < (x5-10) Bankai Byakuya
When you look at it, Ichibei isn’t that much stronger than base Bankai Byakuya (ignoring all the stuff like Ikka Senjinka, Senkai, Shuko Hakuteiken that amp him considerably) given that the gap between Shunsui < Yama and Yama < Ichibe is unquantifiable, all we know is that he is stronger so Kenny amping above Ichibei isnt
This is a really weird argument to make when Kenpachi’s Bankai and Gerard dealt so much damage to him that he fell out of it, meaning he was close to death. Aka he would have been amped massively due to both Zenkai and unlocking more of his own potential since Unohana being weaker than Kenny means that he wouldn’t have achieved his full potential against her. The same guy who gets amps from taking an L and you think he somehow didn’t get stronger?
Kenpachi was also called the strongest Soul Reaper in the TYBW, and Gremmy was called the strongest Quincy initially too. That's why I sort of take things like that with a grain of salt. Ichigo is considered a soul reaper as well (with his Quincy and Hollow powers being stolen and all, though idr if they returned to him by that point), and the statement calling Kenpachi the strongest doesn't say 'Gotei 13 Soul Reaper' but Soul Reaper in general as a blanket statement. I will say it's clear that Kenpachi was above every character that was present in the novel besides possibly Ginjo based on the statements we got, but then again, the god tiers like Ichigo and Aizen, as well as Kenpachi's peers like Toshiro and Byakuya, were purposefully written out of the story. More stock for Byakuya and Toshiro is that, although it was a lie formulated by the Gotei to not get as many people involved in the scenario lore wise, they all found it believe-able to send Byakuya and Toshiro to help
Ichigo out with a threat that they thought would give him trouble. Not saying they scale to Ichigo's full power obviously, but they know how strong he is and felt that they would be relevant in helping him in some way. And we know Hikone is weaker than even casual True Shikai Ichigo based on what the Femritters were saying, with them stating that the only reason Hikone seemed more dangerous was because he was going for lethal blows.
As for Kenpachi's growth post TYBW, Unohana was weaker than him, but not by much. They were relative enough for them to have a long, hard fight even when he was a kid. They weren't leagues apart. But even if you say that, Kenpachi still had his fights against Gremmy, the Femritters, and Perninda before finally fighting Gerard. He would've gotten Zenkai's from Gremmy/Femritters and Perninda before even stepping to the Gerard fight. As for Zenkai's post Gerard, the 'Zenkai' is more like 'When the soul is put into a life and death situation or exposed to mortal danger' rather than specifically being put near death and coming back to life per say. It still applies of course, but taking those words directly, Byakuya and Toshiro (more so Toshiro) had those experiences as well. Worst injury Kenpachi got was losing his arm (which already happened to him earlier), so it wouldn't be an injury that makes him eclipse his previous levels of power like how he did pre Unohana fight. His Amps from taking L's stopped at Unohana where he peaked, and then from there he gains the same gains as anyone else does.
But, if I play devil's advocate and concede that Kenpachi is in fact the strongest pure shinigami, there's not really an evidence that suggests that he far eclipses Toshiro and Byakuya to the point where they're not relative. We know that Shikai Kenpachi was at least 5-10x weaker than Toshiro and Byakuya, perhaps more, depending on if you think TYBW Toshiro and Byakuya were stronger than Bankai Kenpachi. To say that Sealed and Shikai Kenpachi are stronger than Mature Toshiro and Bankai Byakuya would be a huge leap without evidence to support it. If anything, then my list would be mostly the same except having Kenpachi at 6, and then Byakuya and Toshiro right under at 7 and 8. I'd still have him below Gerard, as there's no evidence to suggest he got power that could overcome Final Form Gerard, but I still have the trio over the people from 9 and below for scaling relative to that Gerard in Toshiro and Byakuya's case. I still find it telling that Narita had written the two of them out of the story for the big fight, especially when Byakuya was in Volume 2. TYBW showed the 3 of them as comparable and above the rest for fighting various forms of Gerard, CFYOW shows Kenpachi as strong for being able to fight Hikone and Ikomi, while writing the two that were comparable to him in the last arc out of the story along with Ichigo