Glad that ending sold that for you.
They didn't say it was Nazarick, but there were some subtle hints: namely it was in the "correct" country, it was "unexplored", and there was no mention of it in history. Its almost like... it just appeared lmao
So anyway you arent dumb
Side note, Momon was hired to guard their camp, not raid the tomb with them. So he's not on the hook for em