As long as opinions are free, might as well call you a liar.
At this point we don't know. It possible the manifesto is by the shooter, and he uploaded it to several places himself.
It's possible the maifesto is really by the shooter, and he uploaded it to Instagram, and some random instagram reader grabbed it and sent to 8chan.
It's possible the manifesto is not by the shooter.
It's possible the manifesto never appeared on instagram. I know it appeared on 8chan, saw it there myself.
As for other people knowing the shooter was headed to El Paso to shoot the place up, that's possible too. There haven't been reports of others knowing, aside from reports of multiple shooters, at least one by an eyewitness.
As long as you’re playing “It’s possible...”
Is it possible that Watson is lying about this in an effort to keep 8chan alive?
Why yes. Yes it is very possible.