Is this Rabbinic motif apparently taken from Sifrei Debarim actually between the 3rd to the 5th century and even accepted by Jewish standards?

In Sifrei Debarim, which I am being told is a midrashic compilation apparently from between the 3rd and 5th centuries, we see a Rabbinic analogy for the Jews and the gentiles:

‬‫משל‬ לאחד ששילח את חמורו וכלבו לגורן והטעינו לחמור לתך שלש סאימ .היה החמור מהלך והכלב מלחית. פרק ממנו סאה ונתנו על החמור וכן שני וכן שלישי כך ישראל קבלו את התורה בפירושיה ובדקד וקיה. אף אותם שבע מצות שלא יכלו בני נח לעמוד בהם ופרקום באו ישראל וקבלומ

A man sent his ass and his dog to the granary, where fifteen seʾah [of grain] were loaded atop the ass and three seʾah on the dog. The ass walked and the dog strained to breathe, his tongue lolling. He cast aside one seʾah and placed it atop the ass and then did the same with the second and then the third. This is how Israel accepted the Torah, together with its commentaries and its minutiae. Even those seven commandments that the Noahides could not abide and cast aside, Israel came and accepted.

[Mazuz, H., 2016. Tracing possible Jewish influence on a common Islamic commentary on Deuteronomium 33: 2. The journal of Jewish studies, 67 (2), pp. 291-304.]

Is there anyone familiar with the above metaphor, and if so, can you share whether it actually is a 3rd to 5th century saying or if it is an acceptable quote by Jewish standards?

Thanks.