One of my rebbeim sent a letter to the last Lubavitcher rebbe, asking him about good goyim who keep the 7 mitzvos; if their neshoma is still from klipos tamaeos (שאין להם צד טוב כלל), like the tanya says in perek 1.
He who wrote back to him quoting a sefer in chabad as saying that the neshomos of chasidei umos haolam have the same shoresh as that of yidden, though the neshoma itself is vastly different.
Is this the letter that you’re referring to?
https://hebrewbooks.org/pdfpager.aspx?req=15875&st=&pgnum=56
(Also discussed in the Rebbe’s notes on שיעורים בספר התניא:
https://hebrewbooks.org/pdfpager.aspx?req=19841&st=&pgnum=53&hilite=)