No disrespect intended. I use the Artscroll, too, and IMHO the Hebrew version is loads better than the English.;) I prefer the Hebrew notes and the Hebrew translation to the syrupy editorializing you often get in the English editions.
Being serious for a second, I don't see anything wrong with a world class genius and expert at Talmud using an ArtScroll Gemarah. Even geniuses sometimes like to rummage around in someone else's conciseness. Perhaps the Rav was curious about how a particular editors particular perspective produced a particular explanation of a particular passage. Perhaps he was checking a reference. Even super geniuses are sometimes unable to place a quote, or to remember a cite.
Another thought: Were the learned gentlemen perhaps discussing something that relates to one of the explanatory notes? Or better yet is some new ban being prepared? Did ArtScroll's elucidation cross some a new line? We can only hope. A Rav Elyashiv directed ban on ArtScroll would be AWESOME.