I fully agree with you in that. The construction 'a demand for [something]' is absolutely correct. But what makes me wince is that the [something] here is a subordinate clause with predication!Not if demand is a noun. In which case, 'a demand for [something]' is correct. You can demand (verb) something, but your action is a demand for something. Does that make sense?
If simplified the sentence would be like this:
So too do demands for forms of "proof" will never be observed.
Does it sound normal? I doubt it.