Well, actually no, I’m not mistaken. if tenors are singing from a treble clef without the 8 under it, then the editor made a mistake. The clef shows the transposition. If it doesn’t and the tenors transpose anyway, fine, but it’s not correct music notation. You can find any reference on notation to confirm that fact.
Right. And that’s what I said above. Except that you seem to be confused about the rules regarding clefs. A treble clef with an 8 under it is a transposing clef, down one octave. Put a 15 there and it is down 2 octaves (although that isn’t standard). You can place it above to transpose up as well. Check any notation guide.