Perhaps, or perhaps not, but I think I can debunk the one gemara which was offered as evidence in that thread. It was from Bava Kamma 21a:
אמר רב סחורה אמר רב הונא אמר רב הדר בחצר חבירו שלא מדעתו אין צריך להעלות לו שכר משום שנאמר (ישעיהו כד, יב) ושאיה יוכת שער אמר מר בר רב אשי לדידי חזי ליה ומנגח כי תורא רב יוסף אמר ביתא מיתבא יתיב מאי בינייהו איכא בינייהו דקא משתמש ביה בציבי ותיבנא
The suggestion was that Rav Yosef must disbelieve in the existence of sheidim. But this is not necessarily so. Rashi writes:R. Sehorah slated that R. Huna quoting Rab had said: He who occupies his neighbour's premises without having any agreement with him is under no legal obligation to pay him rent, for Scripture says, Through emptiness (Shaya) even the gate gets smitten. Mar, son of R. Ashi, remarked: I myself have seen such a thing and the damage was as great as though done by a goring ox. R. Joseph said: Premises that are inhabited by tenants keep in a better condition. What however is the [practical] difference between them? — There is a difference between them in the case where the owner was using the premises for keeping there wood and straw.
שנאמר ושאיה יוכת שער - שד ששמו שאיה מכתת שער בית שאין בני אדם דרין בו והלכך זה שעמד בו ההנהו. לישנא אחרינא בית שהוא שאוי ויחיד מאין אדם יוכת שער מזיקין מכתתין אותו:That is, it is a particular demon. Demonology is a complex subject, and it could be that Rav Yosef did not believe in this particular sheid, or that it has this particular effect -- or else had heard a tradition as to the true reason for the introduction of this law.
I will elaborate why I doubt that Rav Yosef would disbelieve in the reality of sheidim in general. If we look in Pesachim 110a:
אמר רב יוסף אמר לי יוסף שידא אשמדאי מלכא דשידי ממונה הוא אכולהו זוגי ומלכא לא איקרי מזיק איכא דאמרי לה להאי גיסא אדרבה מלכא [רתחנא הוא] מאי דבעי עביד שהמלך פורץ גדר לעשות לו דרך ואין מוחין בידו אמר רב פפא אמר לי יוסף שידא בתרי קטלינן בארבעה לא קטלינן בארבעה מזקינן בתרי בין בשוגג בין במזיד בארבעה במזיד אין בשוגג לאI am going to cite Point by Point summary rather than giving my own translation:
Most translations I have seen give this as Yosef the Shed, that is Yosef the demon. Could someone who has conversed with a demon really disbelieve in demons. (We also see, according to the second version, that one who believes in demons could still believe that they would not accomplish some particular action.)
1. Version #1: A king does not normally damage (we need not be so concerned for Zugos; the Beraisos warn against it on account of witchcraft).
2. Version #2: A king does whatever he wants - he may breach a fence to make a path for himself, and people cannot protest (we must be very concerned for Zugos).
1. They strike on account of two whether it was Shogeg or Mezid; they damage on account of four only if it was Mezid.
Now, in William G. Braude's The Book of Legends, I have seen "Yosef Sheda" translated as "Yosef, the expert on demons." That is, "Sheda" is a profession or expertise in demonology, and he is informing these Amoraim about the rules governing demons. Still, that they cite him in any meaningful way indicates that they believe in the reality of demons. (Unless we suggest that Rav Yosef here cites him according to version #2, in order to undermine belief in zugot because he does not believe in their reality.)
Depending on whether מר when used by Abaye refers to Rabbah or Rav Yosef, we may have an explicit example of Rav Yosef saying that something was caused by demons, on Chullin 105b.
I would like to reconsider the gemara in Eruvin about Eliyahu Hanavi or Yosef Sheida. But first, a paraphrase of something from the end of Yerushalmi Terumot:
Thus, sheidim have the ability to transport people from one location to another more or less instantaneously. This is either teleportation or really fast travel. Now, let us turn back to Eruvin 43a:
תא שמע הני שב שמעתא דאיתאמרן בצפר' בשבתא קמיה דרב חסדא בסורא בהדי פניא בשבתא קמיה דרבא בפומבדיתא מאן אמרינהו לאו אליהו אמרינהו אלמא אין תחומין למעלה מעשרה לא דלמא יוסף שידא אמרינהוOr, from the Point by Point Summary:
I would emend this to not just "a certain Shed" by Yosef the Shed, the same one who spoke to Rav Yosef and Rav Papa elsewhere.
1. Suggestion: Eliyahu said them (he flew above 10 from Sura to Pumbadisa) - this shows that Techumim does not apply above 10!Me'iri, Chasam Sofer 6:98
Thus, first the assumption is that only Eliyahu Hanavi could travel that quickly. He would not violate Shabbos by traveling past the techum, so he must have used his other miraculous power and traveled in the air, higher than 10 tefachim. Thus, techum does not apply over ten hand-breadths.
The gemara rejects this because maybe it was Yosef Sheida. That is, Yosef the demon. We see elsewhere that this demon conversed with Amoraim, and could do so here. And demons are not required to keep Shabbos, even if they have the name Yosef! As Rashi writes:
יוסף שידא - דלא מינטר שבתא:
Yosef the Sheid, who does not keep Shabbos. And as we saw from the aforementioned story in Yerushalmi, demons can travel really fast.
Alternatively, if we understand Yosef Sheida as the human expert on demons, then the gemara is suggesting that this human expert was able to harness the power of demons to travel this great distance on Shabbos. And there is no evidence that Yosef the demon expert was a religious man. He didn't keep Shabbos, but still traveled to Pumpedisa and related the seven rulings.
Alternatively, Yosef Sheida is still this human expert, but unlike Rashi's suggestion, he did keep the Shabbos. But travel via demon is instantaneous teleportation, which does not involve travel on the ground or above it.
In any case, the gemara is trying to establish halacha, and relies on the reality of Eliyahu Hanavi moving around among us, and then on the existence and power of demons. This indicates to me that whoever wrote this indeed believed in the existence and power of demons, and was not using it in some metaphorical sense.