(ב) וַיֹּאמַ֗ר יְהֹוָ֞ה מִסִּינַ֥י בָּא֙ וְזָרַ֤ח מִשֵּׂעִיר֙ לָ֔מוֹ הוֹפִ֙יעַ֙ מֵהַ֣ר פָּארָ֔ן וְאָתָ֖ה מֵרִבְבֹ֣ת קֹ֑דֶשׁ מִימִינ֕וֹ (אשדת) [אֵ֥שׁ דָּ֖ת] לָֽמוֹ׃
(2) He said: יהוה came from Sinai, And shone upon them from Seir; [God] appeared from Mount Paran, And approached from Ribeboth-kodesh, from [God’s] right, a fiery law to them.
(ה) ואתה. לְיִשְׂרָאֵל: (ו) מרבבת קדש — וְעִמּוֹ מִקְצָת רִבְבוֹת מַלְאֲכֵי קֹדֶשׁ וְלֹא כֻלָּם וְלֹא רֻבָּם, וְלֹא כְדֶרֶךְ בָּשָׂר וָדָם שֶׁמַּרְאֶה כָּל כְּבוֹד עָשְׁרוֹ וְתִפְאַרְתּוֹ בְּיוֹם חֻפָּתוֹ (שם):
(5) ואתה. And He came therefore to Israel, (6) מרבבת קדש — i.e. and with Him were a some of the myriads of the holy angels, and not all of them and not even the majority of them: not as is the way of a human being who displays all the splendour of his riches and magnificence on his marriage day (Sifrei Devarim 343:11).
(יא) ואתא מרבבות קודש - שלא כמדת הקב"ה מדת בשר ודם. מדת בשר ודם, כשהוא עושה משתה לבנו הוא שמח בחופתו, מראה לו כל גנזיו, וכל אשר לו. אבל מי שאמר והי' העולם אינו כן; אלא מרבבות קודש - ולא כל רבבות קודש.
(11) (Devarim 33:2) "And He came from the myriads of His holy ones": Not as the "measure" of the Holy One Blessed be He is the measure of flesh and blood. The measure of flesh and blood: When one makes a (wedding) feast for his son, he rejoices at his chuppah (his marriage canopy). He shows him all of his treasures and all that he possesses. Not so, He who spoke and brought the world into being. "He came from (i.e., with some of) the myriads of His holy ones," and not (with) all of them.
וְעִמֵּיהּ רִבְבַת קַדִּישִׁין
And with Him were sacred myriads;
Ostensibly, he argues on Rashi in two points:
1. "asa" is a transitive verb meaning He brought, and according to Rashi it is an intransitive verb, meaning He came.*
2. Onkelos seemingly ignores the Sifri's diyuk of the mem prefix** to intimate that Hashem brought only a portion of His sacred myriads. (I made a red mark before the word "ravrivin" in order to stress this omission)
We have been working under the assumption that Onkelos is translating "asa" as "imeih." However, Nesina LaGer proposes that Onkelos isn't translating the word "asa" as "imeih." Instead, "imeih" is entirely Onkelos' addition, which he added in order to more clearly convey the meaning of "rivvevos" in alignment with the interpretation shared by Chazal, Brough by Rashi, that Hashem had "with him" myriads of angels.

**See Rashi later in the passuk (s.v. midaberosecha) where He shows how a "mem" prefix does not necessarily mean "from." See also Mizrachi.
Why indeed didn't Onkelos follow the Sifri's rendering of the passuk with the mem prefix to mean "from", and instead totally ignore the mem?
One approach is offered by Nefesh HaGer. He answers based on a foundational insight into the guiding light of Onkelos in his explanation of the Torah:
Onkelos will sometimes sacrifice the literal translation of the passuk if there is even a a hint of the the passuk showing disrespect or belittling the greatness of Hashem.
This passuk is one of those cases. Had Onkelos translated it literally, there would be two negative connotations of Hashem, Chas v'shalom, ayin sham.

Onkelos isn't arguing in essence with the Sifri as to whether a portion of the angels or all of them is a greater kavod to Hashem or not. Obviously Onkelos does not argue on a Sifri.
Rather, Onkelos does not have the same luxury as Rashi to elaborate and source His commentary, and therefore it can be misconstrued into a disrespect of Hashem by the casual reader, and therefore Onkelos veers from that explanation due to the structure of his commentary.

I think we find a similar disagreement between Rashi and Onkelos earlier in the passuk along the same lines that Onkelos translates a passuk not like a Sifri, not because he argues per se, but because he is always careful to veer away from any intimation that can show disrespect or the perception of Hashem as human or "pedestrian," Chas v'shalom.
(ב) וַיֹּאמַ֗ר יְהֹוָ֞ה מִסִּינַ֥י בָּא֙ וְזָרַ֤ח מִשֵּׂעִיר֙ לָ֔מוֹ הוֹפִ֙יעַ֙ מֵהַ֣ר פָּארָ֔ן וְאָתָ֖ה מֵרִבְבֹ֣ת קֹ֑דֶשׁ מִימִינ֕וֹ (אשדת) [אֵ֥שׁ דָּ֖ת] לָֽמוֹ׃
(2) He said: יהוה came from Sinai, And shone upon them from Seir; [God] appeared from Mount Paran, And approached from Ribeboth-kodesh, from [God’s] right, a fiery law to them.
(ב) מסיני בא. יָצָא לִקְרָאתָם כְּשֶׁבָּאוּ לְהִתְיַצֵּב בְּתַחְתִּית הָהָר כְּחָתָן הַיּוֹצֵא לְהַקְבִּיל פְּנֵי כַלָּה, שֶׁנֶּאֱמַר "לִקְרַאת הָאֱלֹהִים" (שמות י"ט), לִמְּדָנוּ שֶׁיָּצָא כְנֶגְדָם:
(2) מסיני בא [THE LORD] CAME FROM SINAI — He went forth towards them when they were about to take their stand at the foot of the Mount, — as a bridegroom goes forth to welcome his bride, as it is said, (Exodus 19:17) “And Moses brought the people forth to meet God”: this teaches us that He (God) was Himself going forth facing them (cf. Mekhilta d'Rabbi Yishmael 19:17:1 and Rashi on the verse quoted)
(ב) וַאֲמַר יְיָ מִסִּינַי אִתְגְּלִי
(2) He said: ‘‘Adonoy was revealed from Sinai...
