The 'high notes' were in sheet music in Hutchins, "Carols Old and New," 1916, and are the solo notes. Not for us ordinary mortals, or at least not for me!
'Peuple à genoux' are the original words that Dwight changed to 'Fall on your knees.' Adam is credited with the music in this score, but the high solo is not in the Adam sheet music that I found (see below).
Hymns and Carols of Christmas: Minuit chretiens

Sheet music from 1880, orchestration Joncières, shows singable notes (direction Tutti) for 'Peuple à genoux' etc. and shows no solo part: Minuit chretiens
Whether Adam wrote parts or someone else arranged the music for the high solo, I don't know, but it was in the 1916 English score cited above.

The best version of this song that I know is sung by Willie K. (Kalikimaka) on his 1999 Mountain Apple cd (with the high notes!).