S. Stuber, in his article, "מפגשי בראשית – בין אברהם למושלי הארצות", in ספר היובל לרב מרדכי ברויאר (Jeruslaem, 1992): 129-146, identifies these heretics as the surrounding Muslims. They claimed that this story was a forgery inserted into the Torah, for it is inconceivable that a prophet could lie.
A similar statement is repeated here (bolded by me for emphasis):
Polemical motivations – R. Saadia's full-throated defense of Aharon's actions may be partially motivated by an attempt to counter Muslim claims that passages in the Torah which portray sins of prophets are forged insertions.
Is it true that Muslims believe that a prophet cannot sin, and that any bible passage that says so must be incorrect/a later insertion? Please provide sources if possible, and thanks in advance!