Question about the Doubt and Statistics lesson #5

Sh. B. You said (around minute 43) that assuming the normal kashrut of a witness means that we have no indication that he is not telling the truth, and therefore he is trustworthy even in the law of souls, whereas when there is a side to suspect him, such as that he lied a week after his testimony, we rely only on presumption and this is not sufficient for the law of souls. However, with regard to two groups of witnesses who deny the za'za, as stated by Rav Huna 22:32, each of whom comes in a pa'a and testifies, we did not find in the Rambam (Edot 22:1) a distinction between mamunot and nefshot. In the niddushei of the RiM, Yevamot 122:1, he wrote in an explanation and in a simple manner that the za'za is for nefshot.