No it isn't "just" someone who gets "blamed". It's
specifically someone who gets
falsely blamed in order to deflect blame from another.
Definitions.Net:
A fall guy is a person who is used as a scapegoat; someone who ends up taking the blame for the actions of another person or group.
CollinsDictionary.com::
If someone is the fall guy, they are blamed for something which they did not do or which is not their fault.
Moreover, HumblePi clearly used "fall guy" correctly - to mean that Iran was being blamed for something the U.S. had secretly done.
That, after all, is what it means to be "set up" as the "fall guy". Again, it's explicit:
:shrug: etc. so on and so forth.
If you don't like it, don't allow yourself to get glommed onto a position so untenable that apdst can make you look foolish for defending it, and, if you do, quickly apologize, adjust, and clarify your intended position.