According to National Rail Enquiries via the app, there will be a queue system in place and the potential queuing time will be 20 minutes.
Above that disruption notice is another notice saying the queuing time at Waterloo will be 30 minutes. The same at Vauxhall and Wimbledon; 45 minutes at Clapham Junction and 20 minutes at Surbtion.
Either someone mistyped the figure 2 instead of 3 of they are not sure how long the queuing time will be.
The issue comes down to will people heed their existing advice and go on holiday then or work from home? If lots don't travel then queues won't occur quite as much. I'm sure 8 to 8.30 will still be the pinch point as they say.
Now will the 45 minutes of queues at Clapham Junction affect every operator? If I look up Horsham to Clapham Junction, it just states the train will be busy, not that people will potentially have to queue for 45 minutes at Clapham Junction. Perhaps they will have away if just having 45 minute queues that don't affect other operators. Not sure how though, given it will affect multiple South West Trains platforms.
Image of below of the full disruption notices as appears in the National Rail Enquiries App when doing a search for train times on the South West Trains network during the disruption.