The good news is that YES, the Divi Multistep Contact Form plugin works seamlessly with Divi’s native conditional logic.
The Divi Multistep Contact Form plugin was built to be as minimally invasive on the Contact Form module as possible. It still relies on the standard Contact Form’s ability to validate fields and show fields conditionally and does not alter either of these features.
The plugin really just performs one primary function; it allows you to break longer forms into multiple steps that can be shown separately to your users. Every other aspect of the standard Divi Contact Form module is intact and fully functioning.