Looking at Denjiro, I have to agree that he resembles Zoro’s master Koshiro very much.
However, there are several things that make the theory of those two being the same person very unlikely.
First of all, the flashback takes place 41 years ago, and we know that Shimotsuki Kozaburo (implied to be Koshiro’s father) left Wano 50 years ago. So it wouldn’t make sense for him to leave his son behind.
Second, in Koshiro’s Vivre Card, it is said that he was born in East Blue. Again, it’s very unlikely for him to be in Wano after being born in East Blue.
Third thing, in chapter 0 we see Koshiro and infant Kuina in Shimotsuki Village which is in East Blue.
We see them during the time of Roger’s execution which took place before Oden died. And we know he was in Wano fighting Kaido during that incident.
Besides the resemblance, there really aren’t any convincing proofs that Denjiro is Koshiro. I saw many people saying how it was Wado Ichimonji on his back, which I don’t believe to be true because it would be logical for someone to recognize their former comrade’s sword on Zoro. Especially Kinemon, who immediately recognised Shusui.
So in the end I might have been able to find out who Denjiro is.
We saw him back then at Marineford when he reacted to Whitebeard’s arrive and dead.
A lot of people think this guy is the former crew member of the Roger Pirates Scopper Gaban.
*Theory by BuggyDClown