So basically a ‘page builder’ is a drag-and-drop system for pages and posts. This way you can easily define where you want your content. Page builders are theme agnostic. (See for example Visual Composer)
A ‘theme builder’ is usually the theme options. Header/footer colours, sidebars, font’s, etc. With a ‘theme builder’ you can edit the look of your website and sometimes choose different layout styles aswell. See for example the WordPress Customizer under appearance in the WP-Admin.
I hope this makes it a bit more clear. A theme builder usually comes with a theme. While a page builder is for creating the particular layout of pages. (While some themes offer a page builder. A lot are theme agnostic.)