I disagree. He's not teaching "wrong history", he's teaching students to think outside the box and identify the weak points in the fabric of the internet knowledge base. What they choose to do with these skills is up to them - perhaps they will invent a more powerful citation system for fact checking, or even enhance the security of vulnerable financial processes.
"most of it is produced in good faith by enthusiastic (and sometimes mistaken) individuals exercising their free speech" - citation needed. By definition, you will never know what the biggest lies out there are because they're never exposed, so your assumption that "all" lies are childish mistakes is wrong.
What about those who consult (wikipedia, redit, and wherever prof. Kelly is going to strike next) casually? Do you always go back to see that the information you quickly looked up has not been deleted later by some professor playing games with your credulity?
PS. I never said or assumed that 'all lies are childish mistakes'.
There will always be hackers, and there will always be security experts whose job is to keep the hackers out of the system. But you need to think like a hacker to do a good job.
"most of it is produced in good faith by enthusiastic (and sometimes mistaken) individuals exercising their free speech" - citation needed. By definition, you will never know what the biggest lies out there are because they're never exposed, so your assumption that "all" lies are childish mistakes is wrong.