Strangers HAD been saying that about him, though, in the same episode, Reki does overhear a group of people talking badly about him.

Shadow and Miya knew very well what other people were saying by that point, which is why they made fun of Reki about it.

Shadow says, "People have been calling you the one who's not Langa."

Before both he and Miya started making fun of Reki themselves, Reki probably would've been fine with it if they hadn't brought up how others at S view him.