In particular, looking at the cello part from the 1924 Casa Ricorda edition of Respighi's Ancient Airs and Dances (Universal Music Co). Third movement, there's a divisi with both parts playing a G one octave apart. The notes are written normally (not marked as they would if to be played open-string harmonics), but "armonici" is written. My feeling is this is just encouraging us to play sweetly and in perfect harmony. Any ideas?
Edit: FWIW our conductor (Valerie Taylor of Berklee) agrees they should be played as harmonics.