I think you're reading it wrong. You will only earn the Kessel Run medal by finishing the Light Side Half at
Disneyland and Dark Side Half at Disney World. All Coast to Coast Challenge medals are only earned by finishing a minimum half marathon at WDW and a half marathon at Disneyland in the same calendar year. Also keep in mind that while runDisney often gives finisher medals to runners who are swept from the course, they do not give out challenge medals to runners swept from a race. So if you finish the 10K, but are swept from the Half, you will receive the 10K medal, probably receive the Half medal, but will
not receive the challenge medal.
I think you explained it fine.
The motivation to doing the Light Side/Dark Side Challenge as part of the Kessel Run is to get the additional 19.3 mile medal that comes with finishing the challenge. For instance, all Dark Side Half runners received the Kylo Ren medal this year, but only the challenge finishers also received the Death Star medal. Likewise, anyone who finished the Light Side Half and Dark Side Half received the Kessel Run and Coast to Coast medals, but only the runners who did the multi-race weekend challenge received the additional 19.3 miles medals.
Likewise, you can finish a multi-race weekend challenge on one coast and only the Half Marathon on the other coast and receive the Coast to Coast medal/Kessel Run if applicable.
Also bear in mind that the Coast to Coast is earned by finishing a Half at Disneyland and a Half at Disney World in the same calendar year. It doesn't matter which halfs you complete so long as one is at Disneyland and the other is at Disney World in the same calendar year. The Kessel Run and pink Coast to Coast medals are the only medals unique to finishing a specific combination of races.