The issue isn't whether he honestly believed it but that he came up with a rather harsh claim without evidence.

He did have evidence. Perhaps what you mean is that there were alternative hypotheses explaining the same evidence. But the existence of alternative hypotheses does not stop the evidence being evidence. There are always alternative hypotheses, and yet there is evidence. Therefore the existence of alternatives does not prevent something from being evidence.

Consider the following three hypotheses:

A) Nobody was systematically voting up your posts.

B) You were systematically voting up your own posts. 

C) Somebody else was systematically voting up your posts. 

The evidence includes the fact that after having your karma voted down to oblivion, your karma shot up for no obvious reason, allowing you to post a main article to the front page. This evidence decreases the probability of hypothesis A, and simultaneously increases the probabilities of B and C, by Bayesian updating. I don't know what the Popperian account is precisely, but since it is not completely insane then I believe it leads to roughly the same scientific result, though stated in Popperian terms, eg without talking about the probability of the hypothesis. The evidence is in any case inconsistent with A, and consistent with both B and C, or less consistent with A than with B or C, however you want to put it, and thus is evidence for each as contrasted with A. 

As I recall, you yourself proposed hypothesis C. Do you claim that you proposed a hypothesis without evidence, and that this was a bad thing? For that would seem to follow from your own logic. An alternative hypothesis consistent with your own experience is that you did it yourself and just forgot, call this C1. C1 is highly improbable, but to say such a thing is nothing other than to say that the prior of C1 is extremely low, which is Bayesian thinking, which I understand you disapprove of. 

In any case, your complaint that he offered a conjecture in advance of evidence is odd coming from a Popperian. Deutsch the Popperian writes that conjectures are not "derived" from anything. They are guesses - bold conjectures, so he writes. Experience is not the source from which conjectures are derived. Its main use is to choose between conjectures that have already been guessed. So he writes. But now you complain about a conjecture offered - so you claim - in advance of evidence!

And whence comes this new appreciation of yours for social niceties such as holding one's tongue and not blurting out uncomfortable truths, or, since we are after all fallible (as Popper says), uncomfortable guesses at the truth? Did you not claim that we should all be beyond such social niceties, such sacrifices of the bold pursuit of truth to mere matters of "style"? Did you not yourself explain that you deliberately post in a style that does not pull any punches, and that anyone who complains merely proves that they are not worthy of your time? So why the apparent change of heart?