Yes I agree I did not say it never happens, I said it rarely happens and although metformin does not cause hypos it can slow the liver from responding to prevent them. There will always be people that do not fit into the norm.
Generally what you say here holds, but I believe that it is possible to hypo on Metformin only, or even without diabetic drugs at all (i.e. normal non diabetics)
Being a former blood donor once I got a 2.5 mmol/l result that at the time was attributed as a lab error, because was a single event and most other readings were in the low range or sightly lower tha nnormal.