I think you've touched on an important point here, namely that functional programming is a feeling. We know it when we see it! But it's hard to get any two people -- even two domain experts -- to agree upon a common definition.
To me, functional programming is the feeling that you can successfully reason about a local fragment of code, because you know what the code (and its callers, and callees) can and cannot do with the data being passed around. It's functional in the mathematical sense -- i.e., the output is a pure function of the input, no matter how convoluted the definition -- and knowing this helps us to reason more clearly, mainly because we don't have to worry about unseen effects at a distance. So, functional programming is a feeling of confidence achieved by making data transformations as local and as pure as possible.
To me, functional programming is the feeling that you can successfully reason about a local fragment of code, because you know what the code (and its callers, and callees) can and cannot do with the data being passed around. It's functional in the mathematical sense -- i.e., the output is a pure function of the input, no matter how convoluted the definition -- and knowing this helps us to reason more clearly, mainly because we don't have to worry about unseen effects at a distance. So, functional programming is a feeling of confidence achieved by making data transformations as local and as pure as possible.