I think the assertion is that OO objects tend not to live in isolation, whereas functional behavior can. Tons of OO systems need to talk "to the mothership" fo

I think the assertion is that OO objects tend not to live in isolation, whereas functional behavior can. Tons of OO systems need to talk "to the mothership" for configuration, communication instances, live configured singletons & helpers, loaded plugins, services, factories, etc, all which need to be properly initialized and in flight in order to even call anything on the initial object in question. It makes a lot of sense to do broad smoke tests and full-cycle specific behavior tests integrating such strongly dynamically dependent subsystems, than to try to create all this artificial test harnessing and fake objects/servers/connections in order to attempt to isolate testing into object-level "units".