Sometimes they are interchangable but technically they not entirely the same:
- an add-in is a component or app that enhances the softwares functionality and relies completly on the host software environment (ie: cannot run without the host application core functionality)
- an add-on is an app that adds additional features and can work independently from the host software (aka. extensions)
Whether its a mix up or an intended uses of both terms, its not that much of a big deal. Revit has so many inconsistancies that you should be criticizing (loudly all over) rather than wasting time on proofreading Autodesk's online manuals
