The Rebbe never said that. In fact, he says the exact opposite in Sichos Kodesh Shabbos Pinchos year 5741 section 6 based on R. Hillel Paritcher in the name of the Baal HaTanya & the Mitteler Rebbe clarifying the statement in Tanya end of chapter one.
I’ll paraphrase part of the Rebbe’s words: “in reference to the souls of the nations Sefer HaTanya speaks of the idol worshiping majority who’s souls come from the 3 impure husks, however the souls of the righteous amongst the nations who keep the 7 Noahide Laws come from the good of the 4th shining husk and therefore have a place in the World to Come”.
(Nevertheless, the Rebbe does say there that although both the נפש of חסידי אומות העולם and a Jew’s נפש הבהמית come from קליפת נוגה, even a Jew’s נפש הבהמית is בעין ערוך to that of חסידי אומות העולם).
The quotation in the question from Israel Shahak is a mistranslation of Tanya at the end of chapter one which is based on the Ari z”l as cited there. The correct translation of “nefashot umot ovdei gilulim” is "the souls of the IDOLATROUS serving nations” (ovdei GILULIM) emanate from the rest of the impure husks (there is no word for “satanic” in Lashon HaKodesh).
See Likkutei Sichos vol. 13 pg. 230 in the notes.
You can also find this idea in Lessons in Tanya which was edited by the Rebbe himself “It should be noted that among the nations of the world, there are also to be found those whose souls are derived from kelipat nogah. (33) Called “the pious ones of the nations of the world,” these righteous individuals are benevolent not out of selfish motives but out of a genuine concern for their fellow.
(33) See Siddur Im Dach. Shaar Chag Hamatzot 287c; Likkutei Biurim (by Rabbi Hillel of Paritch) 47b”
See חב״דפדיה for an in-depth article.
In-depth shiurim on this topic:
https://m.youtube.com/watch?v=yIZihqVzR0o Minute 27:07
Letter of the Rebbe:
Good Shabbos Menachem
Mr. _ Brooklyn, New York
Greeting and Blessing:
I am in receipt of your letter, in which you refer to a discussion
that you had on the question of whether or not Gentiles have a neshama
. . . .
I cannot see where there could have been any difference of opinion on
this matter, inasmuch as it is explicitly stated in various sources,
and statements are also found in various sections of Tanach (Isaiah
42:5, 57:16), to the effect that Gentiles also have a neshama . . . .
It seems to me, therefore, that you may have been arguing at
cross-purposes, and that perhaps the question related more to the fact
that there are different levels and qualities of soul. Now if this was
the point of contention, then it is true that the soul of the Gentile
and the soul of the Jew differ in their nature, this being connected
with one of the basic principles of the Torah – the fact that the Jews
are a people chosen from among the nations of the world. This
chosen-ness originates in the fact that when G-d was about to give the
Torah at Mt. Sinai, He first offered it to all the other nations of
the world, who refused to accept it. The Jewish people did accept it.
Needless to add, this is in no way inconsistent with the statement of
our Sages, to the effect that the righteous among the Gentiles have a
special status and, according to the Rambam, also have a share in the
Judging by your letter, it is surely unnecessary for me to emphasize
to you what has already been indicated above, namely, that our belief
in the chosen-ness of the Jewish people is not a matter of chauvinism
or fanaticism, but rather the deep-felt realization that this
uniqueness carries with it great responsibilities and special
obligations. This is why, for example, Jews have to fulfill “Taryag
(613) mitzvoth,” whereas Gentiles are not obligated to observe kashrut
and various other restrictions connected with the idea of holiness,
holiness being the essential aspect of the Jewish soul.
If I allude here to the special obligations which are incumbent upon a
Jew, as a Jew, it is only for the sake of emphasizing that what is at
issue is not whether a Jew should meet minimal, or even average
standards, but that it is in fact incumbent upon him to attain the
very maximum. In truth, this is self-evident for, inasmuch as G-d has
given us the innate capacity to attain the very heights of the
spiritual and the holy, it is only right that we learn how to
recognize this incomparable Divine gift; while any lack of effort to
utilize it to the utmost would be in the nature of a derogation. And
if, in one’s business or profession, one always tries to give of one’s
very best – for to do less would be to risk being branded a failure –
how much more so in regard to spiritual capacities, which are of vital
benefit not only to one’s self, but also to one’s surroundings and the
world at large.
With blessing, [Sign.]