The assertion is not that this isn't a sign that a person might harm somebody regardless of their access to the images, or that looking at these images increases their likelihood of harming somebody. Those are big arguments and I have no idea what the data says.

There's a separate issue, which is that it's highly uncivilized and dangerous to imprison people for what is essentially thought crime (the important caveats here being that he did not distribute the images and that they were not produced by harming anybody), but also his behavior is highly disturbing because of the emotional nature of the crime that it might indicate a preclusion to. Those two factors come into very strong conflict.

Personally I think codified punishment of being "disgusting" or "appearing to want to do something illegal" is a vastly greater evil than looking at fake child porn, both morally and practically. Of course, ultimately the people constituting a society decide (indirectly) what is illegal, and their morality-emotions will unavoidably be expressed in laws as far as their framework allows (e.g. the constitution).

However, if enough members of the society align on the same morality-emotions, you can reach a critical mass that allows for changing the framework, which is the real terror I feel when I see stories like this.