Also, it IS a hoax because of the splicing, but the schizos are thinking everyone is calling it a hoax as if to say "the episode itself doesn't even exist."

No.

One newspaper headline containing "hoax" has a subheadline containing "manipulated clips." Another newspaper headline containing "hoax" has a photo caption containing "mash-up clip." They're well aware the episode itself actually exists.

MacFarlane is taking offense probably because people could easily infer from the spliced hoax that he's making a distasteful joke in the aftermath of the Boston Marathon.