Just because you don't understand it doesn't mean it makes no sense.
Currently dwell times are astronomical because there isn't enough room on the platforms to store a trainload of people. So people mostly need to wait downstairs. Once the train arrives it's theoretically safe for them to go to the platform, but they can't because now the stairs are all occupied by the flood of people exiting the train. Then the train needs to wait for all the boarding passengers to make it up the stairs, which takes a while due to the limitation of stairway capacity. And even once they do make it up the stairs, they'll disproportionately enter the train doors nearest to the stairs, which also limits boarding efficiency.
Widening the platforms would allow people to wait on the platforms and stand around each door of the train as other passengers exit, like what you see at a subway station. As soon as those passengers are off, the boarding passengers can make use of the full capacity of every door along the train.
In addition to widening the platforms, Metrolinx is also trying to reduce the passenger turnover at Union. Currently nearly 100% of passengers on a given train get off at Union. They are adding new connections to rapid transit at Mount Dennis (to Line 5), Bloor (to Line 2 via a new tunnel), and East Harbour (Ontario Line), to provide alternate routing options to central Toronto. They are also building additional platforms at Exhibition so they can stop all GO trains there once the Ontario line opens, to encourage as many people as possible to transfer there rather than at Union. Reducing the dwell times at Union also encourages people to use alternate stations by reducing the time penalty for passing through Union. Which in turn reduces dwell time even further.
Currently trains are scheduled with at least an 8-minute dwell, so figure about a 10 minute occupancy including entering and exiting the platform. That's 6 trains per hour per platform. If we can cut the dwell to 3 minutes (which seems totally plausible), each platform could now handle 12 trains per hour.
Prior to the demolition of the south platforms, there were 14 through tracks. Assuming 6 tph/platform that's a total capacity of 84 trains per hour.
That proposed diagram shows 10 through tracks and 4 stub tracks. Assuming the stub tracks are 6 tph, and the through tracks are 12 tph, you get a total capacity of 144 trains per hour, which is a 64% increase in capacity.