You're right that everyone defines spam differently and that you can't give spam a singular objective definition across the board nor was that my intention with what I wrote above. What I did try to say was that every community has a policy of what they define as spam and it's not uncommon for most to share the common view (at least communities I've been apart of) that self promotion is and can be viewed as spam.
Using Twitter as a counter example doesn't fit here because as a company they don't share that view but apparently the subreddit you reference in your original post does
Using Twitter as a counter example doesn't fit here because as a company they don't share that view but apparently the subreddit you reference in your original post does