He tells me...
(And, certainly, it seems, here in the UK, that even MEPs keen on the principle don't really know how this ruling will work in practice or what the wider consequences will be. Video here.
but even if I didn't know, there's a legal concept where ignorance of the law is no excuse, so if there were a law that could get me arrested for mental assault (for lack of a better term)...
You're confusing ignorance of the law (not applicable in your case, you know the hypothetical law) with mens rea (applicable in your case, you did not intend to engage in mental assault).
E = MC ** 2 +- 3db