Yeah, it's an assumption that is very similar to your assumption that Jimmer was his man all along. Bottom line, nobody apart from maybe 5 or 6 people really know the truth in regards to this. Any comments we hear from Petrie, the Maloofs, Westphall or anybody else "in the know", can neither be believed or totally disregarded. Nobody is going to admit that they made a tremendous blunder. So, assuming the Kings management knows deep down inside that they F'd this up, nobody in the organization will ever admit to it, and they will do everything in their power to convince us of the exact opposite, so your assumption is no more logical than mine.
Except that your assumption is that Petrie legitimately had Jimmer ranked higher than Knight, which quite honestly, seems much more far-fetched than my assumption of a gamble that was taken, where things went real bad in terms of a missed opportunity. I mean, I think if we polled the other 29 GM's in the league, and asked them who they had rated higher on their boards, Knight or Fredette, you'd be likely to get about 24 of those GM's rating Knight higher, with maybe 5 of them being really high on Jimmer. The odds would say that Petrie is more likely to be among the 24 than the 5. Of course, that is an assumption as well, but based on just about every mock draft in existence, it's likely within reason.
The getting cute part, is the fact that they "assumed" that they had zero chance at Knight (which I must say, I assumed as much as well), so they felt that they would be fine at 10, because they didn't have a chance at Knight anyways, and they were perfectly content to take Fredette or Walker, whichever one was still available at 10, which they had a good inkling that one of those two guys would still be there.