It might depend when he said it. If it was before he realized it was anything more than gastro? Then he could have been joking.Do you think it’s a possibility that Simon was teasing her when he said is that what you used to poison them?
We don’t know what tone of voice he used but thinking of the timeline, would that fit in with before DC mushrooms were suspected?
Knowing she was a keen forager he might have assumed she’s used foraged mushrooms and accidentally used the wrong ones or teased her about her ability to pick safe ones.
Am I grasping here or is this reasonable?
If it was after his parents died he might have said it in the bitter paranoia of early grief (drawing on my own experience). But that doesn't fit with investigators finding the dehydrator on Friday 4th August. At least, it does, if Erin is lying about him saying it before she dumped it.
Just personally, I can't imagine him saying the thing at all after he knew their lives were threatened but before they died.