The muscular humanoid demons clearly knew how many of his kins had ascended the realms to go to the first demon realm as the procedure was not a simple one and not everyone could do it.
And the jet-black humanoid demon was simply not a part of their entourage.
However, with the jet-black humanoid demon's ability to speak clearly, his appearance that looked more similar to the humanoid demons in the second realm compared to the first realm, and also his ability to use runes, there was simply no way that the muscular humanoid demon would think that the jet-black humanoid demon was a native of the first demon realm.
Especially when he was a runemaster. There was no way for someone to be a runemaster without a guide. And with the resources in the first demon realm, and the lack of any written archives of known runes for the first realm, the possibility of someone being a runemaster was less than 0,1%.