Our Gemara on amud aleph grapples with why relatives are not able to testify. The logical reason is that since they have bias, their testimony is suspect.  The gemara rhetorically asks, “If so, why are Moses and Aaron disqualified from bearing witness for their father-in-law? Could this be because their testimony is not deemed credible? Are we to think that THEY could lie?”  This leads the Gemara to conclude that the disqualification must be understood as “The King’s edict, i.e., a divine decree” that has reasons that we may not be able to fully understand.

The acharonim ask, there is a Jewish jurisprudential principle called, “Anan Sahadi”, which means literally, “We, (the judges), are witnesses.” That is to say, if there is something so plainly self-evident as true, the judges do not require testimony.  If so, what could be more self-evident than the unimpeachable testimony of Moshe and Aharon?  True they are invalidated as witnesses, but they ought to be credible in a different form of evidence, such as anan sahadi.  

Rav Elchonon (Kovetz Shiurim 580) answers, technically this is true.  The Gemara did not raise this point because the status of Moshe and Aharon as witnesses is still different. An anan sahadi can not refute other testimony, while valid witnesses can. Therefore, it is materially and legally significant that Moshe and Aharon are not kosher witnesses even if we “believe” them.

I always look for the simplest peshat, and therefore I would say that anan sahadi only is operative when it is universally self-evident, and not just self-evident to the judges.  Our Gemara that assumed people would find Moshe and Aharon unimpeachable seems to be aspirational, as in fact some members of Moshe’s generation did suspect him of theft and other moral breaches (see Shemos Rabbah 51:6 and Yerushalmi Shekalim 5:2).  Therefore, I would argue anan sahadi would not apply even to Moshe and Aharon. However, this is too easy, Rav Elchonon must have realized this possible answer.  I believe he did not use my answer because of the following Gemara (Moed Kattan 18b):

The verse states: “And they were jealous of Moses in the camp, of Aaron the Lord’s holy one” (Psalms 106:16). Rav Shmuel bar Yitzḥak said: This verse teaches that every man warned his wife against seclusion with Moses because he was jealous. This implies that every man thought that his wife had secluded herself with Moses and sinned, although this was certainly not the case. This demonstrates that it is possible to suspect an absolutely innocent person. 

The Gemara answers: There they did it out of hatred for Moses. They did not actually suspect him of wrongdoing. Instead, their goal was to degrade him by leveling these false accusations against him.

Rav Elchonon can argue similarly, that all the claims of suspicion against Moshe were rhetoric and smear; and the claimants themselves did not believe the propaganda.  A Biblical version of the Russia Collusion Hoax.  I could argue back though, the point of anan sahadi along with all Jewish legal processes, is to settle disputes and maintain law and order. Therefore, a court cannot utilize anan sahadi unless it is so plainly evident that even a cynic would have to admit.  If there was any possible mumble and grumble about Moshe and Aharon, no matter the motivation, it makes their testimony invalid even using the principle of anan sahadi.

Chiddushei Harim (CM 33:4) and Gilyon Maharsha (CM 7:65) argue with Rav Elchonon’s premise. Moshe and Aharon ought to be treated as if they are not to be believed, no matter how credible they actually are.  The Gemara meant that this too is part of the divine decree. 

I have linguistic proof to support the position of the Chiddushei Harim and Gilyon Maharsha. Our Gemara here uses the term “King’s decree” (gzeiras melech) which seemingly is interchangeable with the term scriptural decree (gezeiras hakasuv). As an example, Tosefta Sanhedrin 11:6 and Yersushalmi Sanhedrin 8:1 use the term “King’s Decree” for why a female is exempt from the punishments of ben sorer umoreh, while the Bavli Sanhedrin 69b uses the term gezeiras hakasuv.  It is also notable that the Bavli almost always favors the term gezeiras hakasuv (8 times: Sanhedrin ibid, Bava Metzia 11a, Bechoros 5b, Niddah 22a. Temurah 10a and more), except for two instances when it used gzeiras melech (our Gemara plus Yoma 10a).  Furthermore, it is interesting that there is no “Hey Ha-Yediah”, that is the Hebrew term does not have a prefix letter “heh”, thus it is not THE King’s decree, implying God, but a king’s decree, perhaps just implying a legalism or rule.  The term gezeiras hakasuv also has more divine and specific meaning, as we are referring to a decree from the holy scripture. Therefore perhaps our Gemara deliberately used the term, “gzeiras melech” to imply a general legal principle that had to be applied consistently as many laws must be, even if in this particular instance it may not be justified.  It is less a divine decree than a function of the consistency that is also present in the Torah which requires law to be uniform and broadly applicable.  This supports the position of the Chiddushei Harim and Gilyon Maharsha, as opposed to Rav Elchonon.