Daf Yomi Nedarim 25a: But Isn't the Story About the Snake By King Shapur Also a Guzma?

The Mishna gives examples of vows of exaggeration:

Mishna:

נדרי הבאי
אמר קונם אם לא ראיתי בדרך הזה כעולי מצרים אם לא ראיתי נחש כקורת בית הבד

Vows of exaggeration:
If he said: Konam if I did not see on this road like those who left Egypt; If I did not see a snake like the beam of an olive press.

Shmuel explains the Mishna as referring to as smooth as an olive press, and the narrator of the gemara first compels the explanation of Shmuel by noting that a snake can indeed be that big.

Indeed, snakes can be that big. For example, the largest snake in captivity is Fluffy, a 24-foot python who is as long as a moving van and as thick as a telephone pole. (See here.) I'm not sure of the dimensions of an olive press beam (call for audience participation), but I could well see it rivaling it. And see reports on anacondas.

Yet I am not so convinced that the case the gemara gives to show its existence is a good proof. For the gemara states:

OR IF I DID NOT SEE A SERPENT LIKE THE BEAMS OF AN OLIVE-PRESS. Is this impossible? Was there not a serpent in the days of King Shapur before which thirteen stables of straw were laced, and it swallowed then, all? — Samuel answered: He meant 'as smooth as a bean, etc.' But are not all serpents smooth?

My problem with this is that the number 13, for the stables of straw, might well be a guzma, an exaggeration. Maharatz Chayes makes note that especially where certain classic numbers are involved, a guzma might be in play.

Indeed, here are a bunch of other examples of 13, being used to indicate "a whole bunch":

Shabbat 110a
If one is bitten by a snake, he should procure an embryo of a white ass, tear it open, and be made to sit upon it; providing. however, that it was not found to be terefah. A certain officer of Pumbeditha was bitten by a snake. Now there were thirteen white asses in Pumbeditha; they were all torn open and found to be terefah. There was another on the other side of Pumbeditha, [but] before they could go and bring it a lion devoured it.

Berachot 8a:
R. Ammi and R. Assi, though they had thirteen Synagogues in Tiberias, prayed only between the pillars where they used to study.

Shabbat 119a:
Rabbi Abba bought meat for thirteen istira peshita {each istira peshita = 1/2 zuz} from thirteen butchers and handed it over to them [his servants] as soon as the door was turned and urged them, 'Make haste, Quick Make haste, Quick!' {this was done in honor of Shabbat}

Chullin 8a:
There was a case; Rav Yosef declared up to 13 animals to be forbidden because of a blemish found in the knife after they were all slaughtered.

Chullin 95b:
Shmuel sent him 13 camels laden with (Tosfos - 13 scrolls of) Safek Tereifos (Rashi - alternatively, the Safek (Tamei or Tahor) birds of Perek ha'Tereifos (62B).)

So how can we disprove that something is a guzma (the potential size of a snake) from a story which itself contains elements of guzma?