I guess you miss interpreted the condition.
if $force is false i.e. !$force is true then it means that we don't want to force the copy operation. By this it means that if the destination directory don't exist perform copy, and throw exception.
Only the displayed message is wrong. 
It should be as 'Destination folder already exists'
checkout https://github.com/Achal-Aggarwal/joomla-framework/compare/folder-copy