Not true. This should open a new tab for the add to cart buttons in the free version too. Could you post a link to your site so that we can investigate the issue?
The pro version has more advanced options for opening the links in a new version and includes for example the ability not only to link the actual add to cart button to a new tab but also the links to the product pages (so that the product pages are never shown).
However, depending on the theme used sometimes changes to the settings are required for proper detection of the outgoing links. If you can provide a link to your site we can help with that.