Kaido had the Wano samurai, who are feared worldwide for their strength as affirmed by Brook and demonstrated on-screen by Akainu...
Furthermore, Kaido had the best weapons equipped on his warships, which were incredibly massive. Wano was a weapon factory where Kairoseki originates in this country. Can you imagine the number of weapons loaded on those ships? It's insanely extensive...
He had an army of beasts with artificial Devil Fruits that only standout soldiers among the average can consume. He had 500 of them.
The rest were normal but also strong soldiers. lol
It makes sense for him to be hailed as the strongest pirate, also having an army of giants, with some of their numbers being compared to OARS who wreaked havoc on the crew before the Time Skip, he also has the Flying Six and the calamities in his ranks.