However, you make it sound like it happens all the time. In reality, it happened literally just once (multiple comments, but all within one hour), the edit itself was made obvious on purpose, and it has never happened again (it's been almost 7 years since then).

For additional context to what happened: the CEO was just replacing the his own username (u/spez) in comments on r/the_donald that were just saying variations of "fuck u/spez" with usernames of r/the_donald mods.

Again, I have a very low opinion of Reddit leadership team and their direction, but "CEO editing user comments" is not a concern I have about the website at all. Mostly because it was an isolated incident that happened over half a decade ago.

how do you know exactly?

What concern would there be anyway? Like, who cares if someone edits your comment? It is not like you are storing your important life's work there – or at least I can't see it being a good idea to store your important life work there, even without tampering. Providing long-term durable storage is not the business they are in.