Is it possible to extract the page-builder from Elegant Themes Divi & Extra to build it into your plugin? That sounds to me like it could be quite a challenge. I can understand your plan to restrict your plugin to those themes.
What does the plugin do?
As for testing for the presence of a theme, wp_get_theme() gives all the properties of the active theme, including its name. You just need to test whether the theme name of returned by wp_get_theme() is one of themes you are looking for.
I would imagine your plugin needs to know if the active theme is one of the ones you need, not just that the theme is installed.
Why were you puzzled by this requirement, it seems very straight forward and you seem to know what what you are doing. I feel like I must not be understanding the question.