IIRC, the novel mentioned he could speak the Wookiee language like a native. He certainly seems to be conversing with Chewie at the bar. I don’t see how he could run into Chewie prior to ANH and not encounter Solo though. Other Wooks are possible. Always wanted to see more of Tarrful.
Ben’s familiarity with Mos Eisely means he must have spent some time there or visited frequently. The Cantina is a perfect venue for other characters who are just passing through to appear. I’ve imagined Ben hitting an emotional low point early in his exile and perhaps drinking a little too much.