Depends on how you use those terms. If we're talking about processing power and how sophisticated the underlying tech has to be, then yes I'd tend to agree that the ability to run modern-ish internet-connected apps is at the equivalent of a smart phone. On the other hand, if "feature phone" is about the way the user interacts - a phone that ships with a give set of features and is not intended to run arbitrary apps - then it doesn't matter how sophisticated it is under the hood, only that it bakes in certain apps/features (phone, SMS, Signal) and that's it.
