Zoro could be considered a Samurai because his backstory, yet to be explained, has close ties with Wano. He also parallels Ryuuma who is the Sword God of Wano, was introduced on Wano as a Ronin (a wandering Samurai), and was stated by Oda in an SBS that he represents Japan.
So while he may not be a concrete Samurai, he's being portrayed as such and by the End of Wano he'll be a Samurai in the eyes of Kaido and the general Wano population regardless.
It comes down to semantics really. Does it matter if Zoro isn't a true Samurai if everyone thinks he is and we all know the person Kaido is referring to is Zoro? Compare Zoro to other swordsmen not from Wano such as Law and see how many times they've been called a Samurai.