You can edit a blockchain without introducing new rules. The most obvious way would be to already have rules that allow you to edit it. If someone made a comment containing illegal content, Reddit would need to edit that previous block, they can’t just mark it as deleted in a new block since they would still be hosting illegal content in that case. Another way is a blockchain re-org. If you keep all signed transactions, you can rebuild the blockchain while excluding unwanted transactions. That’s what a 51% attack is essentially used for.
Blockchains don’t have to be public any more than any other databases have to be public. Git repositories are technically blockchains, and are often private.
If reddit made their traditional database public, it would be just as possible to tell any changes they made by comparing it to the previous state. A blockchain would make it a lot easier, but not any more possible.
I think he was talking about reddit keeping karma scores and comments on a blockchain, not about the community points. Just from a practical standpoint, Ethereum would not be able to handle the volume of reddits data even if 100% capacity was dedicated to reddit.
But coming back to the community points, I can nearly guarantee even without looking that it’s a token on ethereum blockchain. Tokens are custom coded, they can and often do have functions for admins such as issuing tokens, removing tokens, freezing addresses and pausing all token transactions. They also very often include an “upgrade” mechanism where the token code can be essentially rewritten.