I figure someone who communicates telepathically would still use his mouth out of habit. It doesnt need to be lip-less like in other series.
Because after an episode that kinda focused on a flashback of one specially upright, likeable, competent classmate, the episode ends with Kumoku surprised about how unusually intelligent the lava dragon is. The impression that gave me was: oh, so that dragon possesses human smartness like Kumoko, that's why he knows how to avoid her poison attack and strategically destroyed the cones that Kumoko used to stand on.
I mean, maybe it's just a smart monster, but I instantly associated that classmate, especially with all the talk about missing classmate. Would be kinda cool if Kumoko found a friend, too.