all impulses are selfish because they're only experienced internally.

I think defining "selfish" as "anything experienced internally" is very limiting definition that makes it a pretty useless word. The concept of 'selfishness' can only be applied to human behaviour/motivations–physical-world phenomena like storms can't be selfish or unselfish, it's a mind-level concept. Thus, if you pre-define all human behaviour/motivations as selfish, you're ruling out the opposite of selfishness existing at all. Which means you might as well not bother with using the word "selfish" at all, since there's nothing that isn't selfish.

There's also the argument of common usage–it doesn't matter how you define a word in your head, communication is with other people, who have their own definition of that word in their heads, and most people's definitions are likely to be the common usage of the word, since how else would they learn what the word means? Most people define "selfishness" such that some impulses are selfish (i.e. Sally taking the last piece of cake because she likes cake) and some are not selfish (Sally giving Jack the last piece of cake, even though she wants it, because Jack hasn't had any cake yet and she already had a piece.) Obviously both of those reactions are the result of impulses bouncing around between neurons, but since we don't have introspective access to our neurons firing, it's meaningful for most people to use selfishness or unselfishness as labels.