How is 那还用说！translated as "Of course!" or "That goes without saying!"?
The point is 用 doesn't mean "use", but "need" here.


  （多用于否定: 需要） need:

There's no need to turn on the light.
不用开灯。

I don't think you need to worry in the least.
我看你一点也不用着急。



And 还 is used for emphasis,


  （用以加强语气）:

That goes without saying.
那还用说!

This is the limit!; This is simply atrocious!
这还了得!



那还用说 is a rhetorical question (echo question), it literally means ”need to say that?!", i.e. "no need to say", then "of course" or "that goes without saying".