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The flyers said "assemble at 1pm". The story went up at 16:30. You can't really write and file a news story while you're in the middle of a march. So - three and half hours to do some marching, find somewhere you can write, write the story, file it, the subs look at it, make sure it's formatted correctly for the web and post it - I mean, it's not super speedy, but its definitely not a turnaround I'd think was abnormal for that kind of news environment.
They did exactly what you suggest - they estimated the numbers and updated later - but that's got everyone's knickers in a twist because the first estimate was too low!
The protest wasn't a flashmob. They could easily have pencilled it into their diaries and done a quick guestimate of attendance a lot earlier and updated the article with further details later.