“Do I say that על פי תורה, this knowledge is forbidden, and for all intents and purposes, I do not know this”
It seems pretty clear to me that the answer is yes. You are not allowed to be mekabel the loshon hora (or to have heard it in the first place), so as far as you are concerned the information is completely false and meaningless.
Therefore, you did not witness the degradation of a Talmid Chacham (and neither did anyone else, if they were in the same situation as you).