You got it right. Capacity for a basketball game is larger. When you have a giant stage facing only 1 way it is greatly reduced. 13,000-15,000 depending on the setup, but it maybe less depending on how much “back stage” area they need for security detail, VIPs, and the like.
Can you imagine the “mess” if they have to switch to the Arena? Chris Wallace was there & said security is “light” because Obama/Biden are never supposed to be there & all the extra security is at the stadium. The SS is going to have to switch all the ‘tight’ security to the Arena. Next, all the “hopeful” Obamabots driving to Charlotte or being bused .... traffic, security, mobs, etc. .... and then it gets cancelled. You know that with such limited room at the Arena & security concerns, they will be letting in only the credentialed delegates. What is everybody else supposed to do? Sit in their cars or on the buses until The Won clears the Arena & area? There are going to be some mad, wet folks. Really mad .... extra, extra disappointed. ;-)