"WB is a living organism, and Kaido is the strongest living organism" yada yada yada, and yes that is what seibutsu means.
Shanks is a sword user and is overall stronger than Mihawk, so these titles need to be taken with a big grain of salt and the context in which they mean.
Primebeard>= Kaido or even >Kaido
Oldbeard+ Grand fleet> Kaido+Beast Pirates
Sickbeard<<Kaido