No, you are right, Hellman does not cite the specific evidential basis for his findings but it cannot be inferred that those findings are not grounded in evidence. Where else, aside from the defence expert, can he have got all that stuff about her mental state? After setting it all out, he says:
However, this Court does not find that there is any significant objective evidence that, when she made her spontaneous statements and wrote her note, Amanda Knox was in not only a situation of considerable psychological pressure and stress but also even in a condition of not intending or wishing; so that, having accused of such a serious crime a person whom she knew to be innocent, she must in any case be held responsible for the crime of calumny, the constitution of which does not require any specific purpose from a psychological point of view, such as shifting the blame off of oneself [conseguire la propria impunità] (an aggravating circumstance alleged [here]). Generic criminal intent [il dolo generico] is sufficient, thus including the aim of escaping from a particularly oppressive personal situation.
I am guessing questions about her mental capacity were put to her expert and that he was not able to go so far as to say she lacked the ability to form criminal intent. We need a transcript of his evidence.