1. Viral tweets have a longer-than-average time window between the time they start to go viral and the time the median viewer sees them, so a community note can get there before the median viewer. 2. Users who interacted with a tweet before it got a community note will get a notification when the community note is added. 3. Community note writers can leave a note on a piece of media. If a tweet with a video gets a community note, and that note is about the video rather than about the tweet, that note will show on all other tweets that show that video.
Source: this excellent interview with the Community Notes team (https://asteriskmag.com/issues/08/the-making-of-community-no...), in the section that starts with "Asterisk: Another thing I wanted to talk about is speed". Really that whole interview is great, highlights how deliberate and thoughtful the Community Notes team was regarding everything about the feature. Which is, I think, why community notes have succeeded where a lot of previous fact-checking attempts have failed.
I'd assume that by the time a disclaimer is written up, submitted, and accepted according to whatever the criteria is, the original un-disclaimered message has been received and digested by its target audience.