Proof from Arvos Moav to Har Sinai regarding who was there

Question

גמרא שבת קמו עמוד א

ישראל שעמדו על הר סיני פסקה זוהמתן גוים שלא עמדו על הר סיני לא פסקה זוהמתן אמר ליה רב אחא בריה דרבא לרב אשי גרים מאי אמר ליה אף על גב דאינהו לא הוו מזלייהו הוו דכתיב (דברים כט, יד-טו)ואת אשר ישנו פה עמנו עומד היום לפני ה׳ אלהינו ואת אשר איננו פה וגו׳

Gemara Shabbos 146a

When the Jewish people stood at Mount Sinai, their contamination ceased, whereas gentiles did not stand at Mount Sinai, and their contamination never ceased. Rav Aḥa, the son of Rava, said to Rav Ashi: What about converts? How do you explain the cessation of their moral contamination? Rav Ashi said to him: Even though they themselves were not at Mount Sinai, their Mazal [guardian angels?] were present, as it is written: “It is not with you alone that I make this covenant and this oath, but with he that stands here with us today before the Lord our God, and with he that is not here with us today” (Deuteronomy 29:13–14), and this includes converts.

Question: The passuk which the Gemara cites as proof that the Mazal [guardian angels?] of the geirim, (converts) were at Har Sinai, is not discussing Har Sinai at all, but rather the gathering at Arvos Moav at the end of the 40 years in the desert. So how can the Gemara use this as proof for what took place at Har Sinai?

Answer 1

Like is explained by the Ba’al HaTurim to Devarim 29:9, Chazal see the same language usage as in Shemot 19:17 and through that make the equation.

אתם נצבים היום גו׳

ויוצא משה את-העם לקראת האלהים מן-המחנה ויתיצבו בתחתית ההר.

There is an amazing discussion of this subject in the introduction to the commentary היכל הברכה by the Komarna Rebbe and properly understanding the nature of why the future generations have any obligation in regard to keeping the mitzvot. It is worth checking out.