Observations

 I spent about 45 minutes this AM....reading/rereading  the full text exchange between Tyler Robinson and his trans-girlfriend/roommate.

Four observations.

First, it just doesn't read like two 20-year old types.  Words used.....I would suggest two 50-year old guys.

Second, in some odd way....it was like you asked AI to create a text between two people who can't read or write.  

Third, in the end.....Robinson asks the idiot friend to delete the text....which any dimwit would figure out....somewhere in the system....it will remain.

Fourth, on my Oswald-conspiracy-index (1-to-10)....I rate the exchange near a '8'.

Finally, just odd.....99-percent of folks would write a manifesto. Robinson?  No.