Why does it say "va'Ydaber Hashem El Yehoshua"? This expression of strong speech is not used for any Navi other than Moshe!
Makos 11a: Here Divrei Torah were taught. Gra
Aruch l'Ner (11a): Even Yehoshua may not teach new Halachos. Surely it was a tradition from Sinai, just since it is written in Yehoshua, it is as if it written in the Torah. Some Amora'im hold that one must be able to fulfill verse 4 (he speaks to the elders) as it is written. We do not find so for any other verse of Nevi'im or Kesuvim!