To the OP:
Think this. Imagine that tomorrow, in middle of this pandemic, future elections, and things like that Chicago is all but destroyed. You are not there. But perhaps you or something you know have family there. And that family survived and tells you all what happened. A big Titan, monsters in the streets. At the same time you read in newspapers, social networks (Supernatural people uses social networks too, except for the wizards), TV, radio tells you that there was a terrorist attack, and some massive allucinations around, probably because of gases or drugs or whatever.
Who would you believe? Wouldn't you think that perhaps the Chicagoan were traumatized for so much slaughter, pain, destruction?
And then I would have said that gangs terrorized people who where in such a state of desperation, panic and pain that they thought they were monsters.
Except this analogy doesn't work because its not as simple as "chicago was all but destroyed." We have a very specific form of attack with far too many witnesses. We dont need to bring the super natural into this. This would be like an asteroid hitting Chicago and the government trying to tell people it was a terroist attack and there was no asteroid. Aint gonna happen. Drugs do not cause consistent hallucinations of the same thing across a group of people that large of that large of an area. There is no chemical weapon that has such effects either. And even if there were, chemical weapons are highly irregular weapons. They would not have effected all of the people in the city, not even close to most of them. What is going to happen is that 8 million people are going to be telling an entirely different story with with a high degree of consistency. It is also impossible for the people of the city to all be in a conspiracy.
"I have an example more on point that WW2 or the nuclear bombs, which were natural. Do you believe the Fatima miracles actually happened? Only around 100 years ago, not 600 or anything, in a time were some cameras already existed and some skepticism were around, literally hundreds of people claim to have seen the Virgin Mary. And more than once. Yes, there are not 8 million people, but there are quite a bunch. And do you, personally, believe it? Probably not, because they told you that they are lying or they had been victims of massive hysteria. Why wouldn't the same logic apply to Chicago?"
No I dont believe in the Fatima BS. People who have religious "hallucinations" (this presumes I dont think they are just liars), already have a concept of thing they claim to see. They also have clearly assumed ulterior motives. Not to mention that their experiences are not repeatable, are generally highly vague. To boot you have that people in the Fatima thing were claiming to see things right after they looked directly into the sun...I could go on but the entire event is entirely nonsense that only a fool could believe. Just like all religious experiences.
This is going to start getting into epistemology, and part of the problem here is that the Dresden Files is already on shaky ground on this regard because the fact that the supernatural world exists and no one is aware of it requires considerable suspension of disbelief. Exploiting this plot device beyond the general hum drum of the detective cases starts to get into absurdity, especially when it was deliberately set up and foreshadowed for the entire novel until it got ret-conned at the end.