I don't know but I have a feeling that Ushimaru knew Dragon. Maybe Dragon took Zoro to Eastblue.
Look at the following panel
Dragon the worst criminal in the world who is renowned around the globe is getting supplies in Shimotsuki village. Furthermore he was a away quite a time just like Ivankov said.
So he was visiting someone, talking to someone. Maybe to Koshiro.
Think about it, everything is tied to Wano, Kozuki Clan, Void century etc etc. There is 0 chance that Dragon knew nothing about the importance of Wano and Oden who travelled with Roger.
Dragon who wants to know about the True History has the visit this place. There's no doubt that Dragon visited Wano.
Knowing Oda, a Panel with Dragons ship and Zoro, that's no coincidence. There is a link between Dragon, Wano and the Shimotsuki Clan especially Zoro and Ushimaru. Fate.
One Piece is all about Fate it was Fate that Luffys and Dragons Father met each other. Bad it is fate that Zoro is Luffys Vice captain.
This reminds at Rogers words to Grap, to take care of Ace.
Add to that Dragons tattoo looks like the symbols on Ryumas Kimono.
I need you all, I need the power of swarm intelligence. What do you guys thinks. Is there a connection?
A certain section in the Eastblue. No we have a certain someone: Ushimaru.
They even talk about food in the Panel. Food is a big Theme in the Wano arc.
Look at the symbolism of the Half moon. Reminds me of what Yuderon the Japanese Youtuber said.