I think you’re getting confused with Theme Forest ‘resizing’ and Theme Forest just fitting on the screen because it’s contained within a wrapping div of some sort.
Upon inspection of Theme Forest’s web page using Firefox and/or Chrome, you’ll they have a div with a class of container and it’s set at 984px, as opposed to the header, navigation bar, footer and sub-footer that is set to be as wide as the browser window itself (100%).
The meta tag line (like the one you posted above) won’t change anything in terms of your physical design, that will have to be done using @media. But what it will do is force the browser on the mobile device to zoom in accordingly.
The meta viewport is not for styling, it’s a signal to the browser/device what the scales should be and assists with media queries by helping determine the width of the device/browser in pixels is.
If you want your website to respond to widths and changes then you will need media queries. Sure you dont absolutely need them and could have just everything percentage based. But if you get to small sizes and want to move your sidebar below the content (for example), you’d need a media query to achieve it.