Except that he didn't tell them about the possible implications of having a psychotic attack. Had he known that, there'd have been no stopping him would there ?
Maybe he didn't go into too much depth because he thought that might arouse suspicion?
On the other hand, he didn't live with Sheila and hadn't throughout the relevant period of time, so there was no reason for him to know much beyond the basics.
To be honest, I don't see the point that the pro-Jeremy side are making here. He knew she was ill. He knew she could 'go crazy'. He knew she was violent. This knowledge does not make him guilty, but it was enough to form the kernel of a plan, if he is guilty.