They...literally do have something to do with them being alive/dead. The other captains and whatnot didn't get this big statement because at the time, they were weaker. No one is scaling first invasion Kenpachi to Yamamoto. And the only person making that statement is dead, so they're not going to make it in cour 2, or 3, or 4.
When Unohana made her statement for Kenpachi, it showed Yhwach who is already considered relative to Yamamoto. In addition, Yamamoto wasn't shown because he's not alive, there would be no reason. Yamamoto himself gets his values from Gremmy, so....
Imo I agree with Arc's scaling. TYBW Bankai Kenpachi is stronger in terms of pure power compared to Bankai Yamamoto, but with his body's inexperience with his power, it's unlikely he would win in a direct fight. Shikai Kenpachi's AP is probably relatively close to Bankai Yamamoto, but only in power. He'd probably get cooked. And if he has his eyepatch on, he'd definitely lose. Basically, in a fight, Bankai Yamamoto>Bankai Kenpachi>Shikai Kenpachi. In terms of pure force, Bankai Kenpachi>Bankai Yamamoto>Shikai Kenpachi.