Good question!
In the Sefer נוצר חסד, a commentary on Pirkei Avot written by יצחק אייזיק יהודה יחיאל זצוק"ל מקאמרנא, (you can find more about him here) he gives an explanation on the first mishnah in Pirkei Avot (1:1) as to what would have been lost/it would not have been correct, by saying that Moshe received the Torah from Hashem. The explanation itself is in hebrew and very lengthy in so l will do my best to summarize his answer:
He explains that the Torah that is received directly from Hashem is not something that Moshe could have passed on. "For every matter there is an interpretation that requires 80 years," as is known from the teachings of the Arizal (haven’t been able to locate the source). In the Sefer עמק המלך it brings a story about Rabbi Avraham Halevi, who found the Arizal speaking in his sleep. When asked if the Rabbi would reveal what he had learned, the rabbi began to laugh and said, "I bear witness to heaven and earth, if I had taught for eighty consecutive years, I would not have been able to complete what I learned this time in the portion of Bilaam and the donkey." This Torah, which came from Hashem, is called the Torah of Atzilut (see here for explanation), which means a very high level of Torah learning (since Atzilut is the highest spiritual world) and to interpret that Torah and explain it, requires eighty years to convey what was received in one moment during sleep.
To make this make more sense, a comparison can be made to the contemporary reality where vast amounts of information can be transmitted with the push of a button. Similarly, although there is a vast distance between this and the Torah of Atzilut, the Torah is given from the mouth of Hashem in such a way that one who receives it needs 80 consecutive years to transmit even one part of what was received in a moment of sleep. The Arizal explains that any righteous person who reaches the state of אין (see here for explanation), meaning a very high level in spirituality, can merit receiving the Torah of Atzilut. The Baal Shem Tov also said that he knew righteous people who attained the state of אין and received Torah from the mouth of Hashem, yet they did not believe in themselves until their death. Consequently, this Torah of Atzilut that came from the mouth of Hashem is not something Moshe could have received from Hashem and passed on to Yehoshua. So until now it's just משה קיבל תורה מהקב״ה, period.
However, after what happened on the 17th of Tammuz with the sin of the golden calf and the breaking of the tablets, the manner of receiving the Torah was different with the second tablets. The Torah was given in a way that Hashem descended upon Har Sinai, which is considered the Torah of Briah (see here), meaning a level of Torah learning which is a step down. This is the Torah that was given to us, which Moshe received from Sinai and passed on to Yehoshua. Indeed, Moshe, who reached the level of אין, received the Torah from the mouth of Hashem. However, what he passed on to Joshua was only the Torah of בריאה– the Torah from Sinai. Moshe did not pass on to Yehoshua everything he received, since for example מה שתלמיד ותיק עתיד להורות/לומר/לחדש לפני רבו (see here) was given to Moshe but not Yehoshua and onwards. But rather he passed on the Torah of בריאה.
Therefore in summary, the reason it says משה קיבל תורה מסיני is because it refers to the form of receiving the Torah on the second tablets, which were given in the manner of Sinai, which is the aspect of Torah that comes from בריאה which was passed down from Moshe to Yehoshua and not אצילות which is the highest form which was only able to be passed down from Hashem to Moshe and includes a lot of Torah that we don't know.
