Plus they did have a common language that both of them "spoke" fluently.
The story did imply that Guren could speak the language because he was a scholar. It's likely that most people of his race had no ability to use this language.
Remember, the two races communicate and move in very different speeds; Guren was chosen specifically because he was the slowest moving of his race, and it is clear he was also one of the most patient. And you must also account for the difference in perception - for the snails, it was probably incredibly obvious that the sand was made of the other people. They probably never realized the other people did not know. And Guren's race never knew there was anything worth investigating.
I think the situation is sort of like me and my next door neighbours - I don't dislike them, I just don't really know them. I see them occasionally if we happen to leave the house at the same time, but I don't say much beyond a polite hello, nor do they. I also know they are of East Asian extract, probably Korean or Chinese. I certainly wouldn't start a conversation with them in order to point out that they are Asian; I figure that they know it, and even if they somehow didn't, why should they hear it from me? And if they are not aware of this fact, I doubt they'll ever bother asking me what ethnicity they are.
This is not a matter of dislike or superiority - I feel neither towards them, and I doubt they feel either towards me. It's a matter of ambivalence. Just because I see someone on an occasional basis and I am perfectly capable of communicating with them doesn't mean I have any interest in doing so or vice versa.