I don't think that's quite right. It was not that he did not know he possessed them, it was that he did not posses them even though they were in his browser cache
That's a great theory, but it doesn't seem to match up with what the ruling actually says:
We must consider, among other issues, the evidentiary significance of "cache files," or temporary internet files automatically created and stored on a defendant's hard drive, and the defendant's awareness of the presence of such files. We conclude that where the evidence fails to show that defendant had such awareness, the People have not met their burden of demonstrating defendant's knowing procurement or possession of those files.
I interpret this to mean that if the defendant didn't know about the cache, the claim that the cached files (all by themselves) are contraband is dead in the water. But I am not sure they are saying that a user who knows about the cache becomes the possessor of a web page the instant it appears on his screen. Look at what they said later:
We also agree that where a promotion or possession conviction is premised on cached images
or files as contraband, the People must prove, at a minimum, that the defendant was aware of the presence of those items in the cache.
We hold, however, that regardless of a defendant's
awareness of his computer's cache function, the files stored in
the cache may constitute evidence of images that were previously
viewed; to possess those images, however, the defendant's conduct
must exceed mere viewing to encompass more affirmative acts of
control such as printing, downloading or saving.
This is a little confusingly worded, but the majority seems to want to see proof that the defendant tried to keep the images by storing them elsewhere on the hard disk. The dissent wants to see proof that the defendant wanted to keep the images on his screen. Both seem to see the cache (at least when used as intended) more as part of the trail of evidence which shows what buttons the defendant pressed in his web browser. They seem to want to see proof that the images were present on his computer by his orders and not simply as a side-effect of the technology.