There is another explanation..... this could be classic post hoc reasoning.
One journalist is informed early on of (presumably) one policeperson's theory, which becomes, " How could they have reached such a dramatic conclusion just a day after finding Meredith's body?"
This is not evidence that "they" had come to this conclusion. It is also not a stretch to imagine that a single cop thought this, told a reporter, and the reporter simply noted it.
With all due respect, this shows the smoke but fails to pinpoint the fire.
I don't really think so. Especially as the reporter states explicitly that upon receipt of this information, Pisa called a senior policeman, who "confirmed" that the police were questioning Knox. And note that the article specifically states that "the police had a suspect". Not "one policeperson said that he privately thought Knox might be a suspect". The construction "the police had a suspect" clearly implies that this was the prevailing view of the investigating team.
It would certainly be interesting to hear from other relevant parties here - particularly the Italian journalist who apparently told the UK reporters about the police suspecting Knox, and other journalists (UK, Italian or American) - to find out just how much traction this really had at the time. Were other journalists let in on this information? If so, when?
I certainly would not be one little bit surprised if one or more puffed-up senior Perugia police officers - excited at being in the centre of a global news story, and more than a little keen to bask in their own glory - eagerly talked to their media chums (the ones to whom they were undoubtedly regularly leaking inside information about the common-or-garden criminal scene in Perugia) about how quickly they'd latched on to Knox. Remember, they all sincerely thought (IMO) that they'd really found the culprit - so it's easy to see how they might have wanted to communicate this "under the radar" to the media, to demonstrate their prowess and speed in "solving the crime".
I just don't think, based on the apparent events that were unequivocally detailed in this article (the Italian journalist informing them on 3rd November that Knox was being considered a suspect, and Pisa's call to a "senior detective", implied to be on the same day or perhaps the day after, in which the detective confirmed the information), that this can be down to either post hoc reasoning or wilful deception on the part of the author.
In passing, I happen to believe that Mignini would never have leaked information of this sort to the media (I believe any such leak would have come from within the police). If Knox really was a suspect in the investigation as early as 3rd November (and I very strongly believe that she was, for many reasons, of which this article is but one), then Mignini too would have been privy to this information - indeed he might have been instrumental in casting suspicion onto Knox. But I think Mignini's stock in trade is more subtle and cunning than to tip the media off about suspects. I think his modus operandi was always to plant little seeds here and there - in this particular regard, for example, he might very possibly have been leaking bits of adverse information about Knox with a metaphorical nod and wink. But I don't believe Mignini would ever have told anyone in the media that Knox was explicitly being considered a major suspect.