You are quoting Sam or me? I didn’t say anything about hidden or dormant. Actually, I would have answered the Tosafos Yeshanim’s differently.
To recap my point, Al taamin does not justify saying a Tzadik did a Chet, but it would justify going sour. It can apply to this Talmid, but I didn’t realize you meeant it about him.
The fact that, according to this story, the Choseh was tricked or led to believe is a hard pill but in light of Yehoshua bin Nun and Achiya Hashiloni being tricked we see that it is not that simple. Rebbeinu Saadya Gaon says that Hashem purposely wanted Neviim to be fooled so that we see that it is Nevua from Hashem and not a special sense.
Nor does this mean that the Talmid became a Rasha. He probably meant what he said. The Kotzker had a nuanced Derech that is hard for binary thinking people to handle.