Well, if it was fabricated as you allege, Jacob502 should have no trouble appealing and overturning the MBW ban.
I'm not a mod or admin at MBW, and the PM wasn't sent from or to me, so I can't state with certainty that Jacob502 definitely sent it.
However, I will say, it sure sounds exactly like one he would send. Did you read the PMs he sent to LZH? Additionally, when Jacob502 posted to Sticky that he'd fly him halfway around the world for 3,000 rep points, I jokingly posted "do you guys smell something? It smells like desperation." or something pretty close to that. Jacob502 didn't say anything in the thread... instead, he sent me this PM:
I didn't bother to reply... it would be fruitless. But, does that seem like a reasonable response to a joking post about someone being desperate for rep points?
EDIT: SLS AMG - I just figured something out. When I received the PM from Jacob502 here - foul language isn't censored... the last word in the first sentence starts with a "c" and ends with a "t" - I'm sure you can figure it out. When I cut and pasted it into a post for the public forum, the vbulletin software automatically censored it. So, if MBW's software works the same, the screenshot of the PM with the words not censored, could easily be possible if public posts are censored but PMs aren't (just like here).
On MBWorld even words do get censored in private messages
World Fastest (NA) SLS AMG in a Standing 1/2 Mile
Trap Speed 254.81 Km/hr Qatar Mile- NOV, 30th 2013