Those are indeed real comments by real people, but usually if they show up on your replay of the map you played, it actually would mean those were all the comments from multiple different replays that go into the same map. The majority of those will be for the replays on the top of the leaderboard, so if you see a popular map where tons of professional or well-known players have made plays on, you'll see a lot of those comments may be referring to them.
I never really understood the discussion/comment system myself when it came to replays, and it can get a little annoying whenever i play anything popular.