I believe the total number of attendees was 150k. The same thing happened with student loans protest where the police go in very hard and manage to produce the arrests required. But the vast majority of people do not go to commit violence. Same thing happens at pro-Palestine protests and every other protest (although due to the funding situation and the executive structure of the Met, the policing guidance given to those protests tends to be different).

And btw, yes it is possible the Met are lying. There have been multiple FOIs in the recent past where police forces have lied about this kind of thing. Aston Villa policing being one of the most obvious examples. It isn't particularly relevant to this case though but, given the massive changes in policing over the last few decades, it is generally wise to be cautious. This is why the IOPC exists and has been so critical of the Met.