Why did Shmuel say this?
Radak: Hashem, who prepared Moshe and Aharon to redeem you, and through them took your fathers out of Egypt, is the witness between us (like verse 5 said).
Malbim: The voice you heard say "Ed" is Hashem, who made Moshe and Aharon. He testified that my righteousness is like theirs.
Why does it mean that Hashem 'made' Moshe and Aharon?
Rashi: He made them proper in their mission, to take your fathers from Egypt.
Radak: He aggrandized them and taught them. This is like "v'Es ha'Nefesh Asher Asu v'Charan" (Bereishis 12:5).
Radak citing Targum Yonasan: He did mighty acts through Moshe and Aharon.
Malbim: He prepared them to be Shoftim over Yisrael.