Safari may pretend to have dev tools that actually work — but don’t be fooled, it is possibly worse than Internet Explorer at this point. If you use the developer menu and select ‘disable caches‘, this checkbox will literally do nothing.
If you click into Safari > Reset Safari and select Remove All Website Data, you have a 50/50 chance of this also doing nothing. To get as close to a guarantee as possible that Safari will actually clear a cache when you want it to, you will need to go through the Reset Safari process, close the window of interest, and start over in a new window.