Daf Yomi Ketubot 20a-b: Does Rav Ashi Act Against His Own Pesak?

Ketubot 20, about whether a litigant may help a witness recall the testimony:

איבעיא להו לעצמו מאי

פירוש אם שכח העד מהו שיזכירו בעל דין
רב חביבא אומר אפילו לעצמו מדכר
רב אשי אמר לעצמו לא
והלכתא לעצמו לא
ואי צורבא מרבנן הוא אפילו לעצמו כי הא דרב אשי הוה ידע ליה סהדותא לרב כהנא אמר ליה מי דכר מר לההוא סהדותא
א"ל לא אמר ליה ולאו הכי והכי הוה לא ידענא לסוף אידכר רב אשי אזל אסהיד ליה חזיא לרב כהנא דהוה מחסם א"ל מי סברת עלך קא סמיכנא אנא הוא דרמאי אנפשאי ואידכר:

It was a question to them: For himself, what?
To explain: If the witness forgets, may the litigant {himself} remind him?
Rav Chaviva says: Even he himself may remind.
Rav Ashi said: For himself, no.
And the halacha is that for himself, no.
{Ketubot 20b}
And if he is a Torah scholar, even for himself. Like this of Rav Ashi in which he knew evidence for Rav Kahana and he {=Rav Kahana} said to him, "does Master remember the testimony?" He said to him "no." He said to him, "and was it not such-and-such?" "I don't know." In the end, Rav Ashi remembered and went to testify for him. He saw Rav Kahana that he was surprised. He said to him, "do you think that I am relying upon you? I was the one who wracked my brains and remembered."

So goes the gemara. According to this, Rav Ashi first says that a litigant cannot. Then, a story involving Rav Ashi himself appears to contradict it. The anonymous setama digmara preempts this question of maaseh listor by introducing it with the equivalent of adam chashuv shayni.

Who is the tzurba meirabbanan? It could be Rav Kahana the litigant, who we might now trust that he is not trying to unduly influence the testimony. Or, much more likely, it is Rav Ashi, the witness, who just ruled one could not rely on the litigant.

Indeed, it is hard to see how this is a maaseh listor. After all, Rav Ashi says that he was not relying on Rav Kahana, the litigant. Rather, he wracked his brains until he recalled it by himself.

This may be the intention of the gemara with its exception of tzurba meirabbanan. We can trust the Torah scholar -- he is granted neemanut -- that he will not be unduly influenced, such that the litigant's prompting will not exclude it. He, unlike other witnesses, will not testify if he has the slightest doubt about the matter, and so will only rely upon his own recollection in the end. The prompting by the litigant will only prompt him to dwell on the matter until he can or cannot remember.

Thus, in reality, a tzurba meirabbanan is no different, in that even he may not rely on the litigant. He is different in that we are confident that he is not relying on the litigant.

This is essentially Rashi, but possibly slightly different and with slightly different stress.