Links to the studies please hold.
Seriously, you are correct. Sadly you'll probably not create any converts.
It was very likely someone who actually speaks/writes just like the ransom note. While it is presumptively presented that the author modified or disguised their writing or grammar when writing the ransom note, its not a known that this is the case. Nobody can show that the ransom note is not an exact copy of someone's handwriting, grammar, and style.
Another poster had links to a linguistic study of the ransom note. In the words of the linguist, the ransom note was plenty long enough to produce a unique style that belongs to one person. PR was ruled to be incapable of creating the note because she never demonstrated most of the unique style-markers that are present in the ransom note.