I really don't get this approach.
Because you seem to be misunderstanding what people are objecting to.
No one as far as I have noticed has suggested the law which says people can self-d in terms of toilet use is a good idea. And pretty much everyone agrees that it's somewhat predictable that something like this could happen in these kinds of situations.
What people are saying seems to be broken down into three things:
People pointing out that as long as self-id is the law, your point that this is not a space for biological males and everyone should know this is inaccurate. Many people really do believe that gender identity is a real thing, and given that that the law upholds them in this belief, it is clearly not obvious to some people, maybe quite a few.
People pointing out that the earlier posts made making statements that the victim intended to be threatening, and knew that their presence would be disconcerting, and that was in fat the purpose of being there, are not something we could possibly know. There are no indications of that in the information we were given, and there are all kinds of other scenarios that are possible and even likely. Especially considering this individual has been told, including by the authorities, that using the women's facilities is ok.
People who feel that pointing to actions of a victim that may have contributed to their victimisation is always in bad taste and amounts to victim blaming. Personally I don't think that is always the case, sometimes those conversations are appropriate. But many people who post here are pretty militant about it, and so why be surprised when they apply it to other types of violent crimes.