Well the reason this plugin is called ‘Bulk Delete’ is because this plugin allows you to delete posts/pages/uses/post meta etc in “BULK”. This is what the free plugin does.
If your server is able to handle the load then you don’t have to buy the add-ons at all. Even if you have thousands or millions of posts, the free plugin itself is perfectly capable of deleting all of them in a single go. The only limitation is your server’s resources. There is nothing in the plugin code that prevents the number of posts or users that can be deleted.
You buy the pro add-ons only when your server is not able to delete all of them in one go because of resource constraints and you want to delete them in batches by scheduling them.
Hope this clarifies the reason why I am calling it ‘Bulk’. Let me know if you still have any other questions.