No, you can't just "edit the chain." You'd need to fork it with new rules to have it accept whatever arbitrary transaction you wanted to insert. Furthermore, even if the blockchain was set up in such a way as to make that easy, it would be obvious to everyone what had happened. The blockchain is a public database. Reddit's back end is currently a private database. If Reddit changes a karma score or whatever over on their current private database, how can you tell? How can you prove it? If they try to do something like that with a blockchain everyone will see it.
Assuming it even works - you speak of "controlling the majority of computers" as if it's something that's easy to do. Reddit's "Community Points" tokens are on the Ethereum blockchain. Under its current consensus algorithm you'd need to control 66% of the stake. The current amount staked is 21921671 Ether, which at current prices is 41.4 billion US dollars. You would have to buy on the order of 50 million Ether to overcome the existing stake, and there simply isn't that much available for sale so the price is literally incalculable. They've made it a very hard blockchain to break.